Use analytic springs and instrument resize barriers

Share a stable damped oscillator solver across scalar and rectangle animations while preserving retarget and explicit no-crossing presentation policies. Add deterministic traces, scrolling and zoom continuity coverage, and pause-aware advancement.

Measure resize barrier and handoff timing behind resize-metrics, expose the feature through the release installer, and wait for the intended backend in isolated portal tests.

compositor/Cargo.toml

@@ -29,3 +29,7 @@ xcursor.workspace = true
 
 [dev-dependencies]
 tempfile.workspace = true
+
+[features]
+# Opt-in counters/histograms with a per-client exit summary.
+resize-metrics = []

compositor/src/backends/direct.rs

@@ -660,6 +660,7 @@ pub(crate) fn sleep_locked_outputs(state: &mut Ferese) {
 }
 
 pub(crate) fn wake_locked_outputs(state: &mut Ferese) {
+    state.reset_animation_clock();
     if let Some(backend) = state.direct_backend.as_mut() {
         for device in backend.devices.values_mut() {
             for output in device.outputs.values_mut() {

compositor/src/main.rs

@@ -22,6 +22,8 @@ mod presentation;
 mod private_client;
 mod reload;
 mod render;
+#[cfg(feature = "resize-metrics")]
+mod resize_metrics;
 mod resize_transaction;
 mod session_lock;
 mod shell_control;
@@ -143,6 +145,9 @@ fn main() -> Result<(), Box<dyn Error>> {
         }
     });
 
+    #[cfg(feature = "resize-metrics")]
+    state.resize_metrics.dump();
+
     runner.borrow_mut().stop();
     if let Some(child) = &mut child {
         terminate_child(child);

compositor/src/overview.rs

@@ -3,7 +3,7 @@ use std::sync::{Arc, Mutex, mpsc};
 use std::time::Duration;
 
 use ab_glyph::{Font, FontArc, FontVec, ScaleFont};
-use ferese_animation::{AnimatedRect, AnimatedValue, SpringConfig};
+use ferese_animation::{AnimatedRect, AnimatedValue, CrossingPolicy, SpringConfig};
 use ferese_core::{OutputId, WorkspaceId};
 use ferese_layout::{Direction, Rect, WindowId};
 use smithay::backend::allocator::Fourcc;
@@ -1016,7 +1016,7 @@ impl OverviewMotion {
 
         for presentation in self.presentations.values_mut() {
             if animations_enabled {
-                active_animation |= presentation.advance(delta, spring);
+                active_animation |= presentation.advance_with_policy(delta, spring, CrossingPolicy::NoCrossing);
             } else {
                 presentation.snap();
             }
@@ -1029,13 +1029,14 @@ impl OverviewMotion {
         if animations_enabled {
             // Geometry's 0.1-pixel tolerance is far too coarse for opacity:
             // it would abruptly drop the last ten percent of the fade.
-            active_animation |= self.opacity.advance(
+            active_animation |= self.opacity.advance_with_policy(
                 delta,
                 SpringConfig {
                     position_tolerance: 0.001,
                     velocity_tolerance: 0.005,
                     ..spring
                 },
+                CrossingPolicy::NoCrossing,
             );
         } else {
             self.opacity.snap();

compositor/src/resize_metrics.rs

@@ -0,0 +1,241 @@
+//! Opt-in resize diagnostics. No measurement code exists without resize-metrics.
+//! Times are unscaled wall time except handoff_animation, which is the existing
+//! handoff clock (affected by animation speed and pauses).
+use std::collections::HashMap;
+use std::time::Duration;
+
+use ferese_core::WorkspaceId;
+use ferese_layout::WindowId;
+
+const BUCKETS_MS: [u64; 16] = [16, 32, 64, 75, 80, 85, 100, 150, 250, 290, 299, 300, 301, 310, 350, 500];
+
+#[derive(Debug, Default)]
+struct Histogram {
+    counts: [u64; 17],
+}
+
+impl Histogram {
+    fn add(&mut self, duration: Duration) {
+        let index = BUCKETS_MS.partition_point(|ms| Duration::from_millis(*ms) < duration);
+        self.counts[index] += 1;
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) enum End {
+    Commit,
+    Deadline,
+    Replaced,
+    Cancelled,
+}
+
+#[derive(Default, Debug)]
+struct Client {
+    workspace: Option<WorkspaceId>,
+    app_id: String,
+    started: Option<Duration>,
+    target_changed: Option<Duration>,
+    last_observation: Duration,
+    handoff_started: Option<Duration>,
+    barriers: u64,
+    deadlines: u64,
+    commits: u64,
+    replaced: u64,
+    cancelled: u64,
+    viewport_pauses: u64,
+    other_window_pauses: u64,
+    viewport_seen: bool,
+    others_seen: bool,
+    viewport_overlap: Duration,
+    other_overlap: Duration,
+    barrier_duration: Histogram,
+    viewport_blocked: Histogram,
+    other_windows_blocked: Histogram,
+    target_to_handoff: Histogram,
+    handoff_wall: Histogram,
+    handoff_animation: Histogram,
+}
+
+#[derive(Default)]
+pub(crate) struct ResizeMetrics {
+    clients: HashMap<WindowId, Client>,
+    workspace_started: HashMap<WorkspaceId, Duration>,
+    workspace_duration: Histogram,
+}
+
+impl ResizeMetrics {
+    pub(crate) fn begin(&mut self, id: WindowId, workspace: Option<WorkspaceId>, app_id: &str, now: Duration) {
+        // Keep an overlapping workspace barrier continuous through retargets.
+        let previous_workspace = self.end_client(id, now, End::Replaced);
+        if previous_workspace != workspace {
+            self.finish_workspace(previous_workspace, now);
+        }
+
+        let client = self.clients.entry(id).or_default();
+        client.workspace = workspace;
+        client.app_id.clear();
+        client.app_id.push_str(app_id);
+        client.started = Some(now);
+        client.target_changed = Some(now);
+        client.last_observation = now;
+        client.handoff_started = None;
+        client.viewport_seen = false;
+        client.others_seen = false;
+        client.viewport_overlap = Duration::ZERO;
+        client.other_overlap = Duration::ZERO;
+        client.barriers += 1;
+        if let Some(workspace) = workspace {
+            self.workspace_started.entry(workspace).or_insert(now);
+        }
+    }
+
+    fn end_client(&mut self, id: WindowId, now: Duration, reason: End) -> Option<WorkspaceId> {
+        let client = self.clients.get_mut(&id)?;
+        let started = client.started.take()?;
+        client.barrier_duration.add(now.saturating_sub(started));
+        client.viewport_blocked.add(client.viewport_overlap);
+        client.other_windows_blocked.add(client.other_overlap);
+        match reason {
+            End::Commit => client.commits += 1,
+            End::Deadline => client.deadlines += 1,
+            End::Replaced => client.replaced += 1,
+            End::Cancelled => client.cancelled += 1,
+        }
+        client.workspace
+    }
+
+    pub(crate) fn end(&mut self, id: WindowId, now: Duration, reason: End) {
+        let Some(workspace) = self.end_client(id, now, reason) else {
+            return;
+        };
+        self.finish_workspace(Some(workspace), now);
+    }
+
+    fn finish_workspace(&mut self, workspace: Option<WorkspaceId>, now: Duration) {
+        let Some(workspace) = workspace else { return };
+        if !self
+            .clients
+            .values()
+            .any(|client| client.workspace == Some(workspace) && client.started.is_some())
+            && let Some(started) = self.workspace_started.remove(&workspace)
+        {
+            self.workspace_duration.add(now.saturating_sub(started));
+        }
+    }
+
+    pub(crate) fn observe(&mut self, id: WindowId, now: Duration, viewport_active: bool, other_active: bool) {
+        let Some(client) = self.clients.get_mut(&id) else {
+            return;
+        };
+        if client.started.is_none() {
+            return;
+        }
+        let interval = now.saturating_sub(client.last_observation);
+        client.last_observation = now;
+        if viewport_active {
+            client.viewport_overlap += interval;
+            if !client.viewport_seen {
+                client.viewport_seen = true;
+                client.viewport_pauses += 1;
+            }
+        }
+        if other_active {
+            client.other_overlap += interval;
+            if !client.others_seen {
+                client.others_seen = true;
+                client.other_window_pauses += 1;
+            }
+        }
+    }
+
+    pub(crate) fn pause_clock(&mut self, now: Duration) {
+        // Activity overlap excludes globally paused presentation time. Barrier
+        // and target-to-handoff durations remain actual wall-clock durations.
+        for client in self.clients.values_mut().filter(|client| client.started.is_some()) {
+            client.last_observation = now;
+        }
+    }
+
+    pub(crate) fn handoff_tick(&mut self, id: WindowId, now: Duration, blocked: bool) {
+        let Some(client) = self.clients.get_mut(&id) else {
+            return;
+        };
+        if !blocked && client.handoff_started.is_none() {
+            client.handoff_started = Some(now);
+            if let Some(changed) = client.target_changed.take() {
+                client.target_to_handoff.add(now.saturating_sub(changed));
+            }
+        }
+    }
+
+    pub(crate) fn handoff_end(&mut self, id: WindowId, now: Duration, elapsed: Duration) {
+        let Some(client) = self.clients.get_mut(&id) else {
+            return;
+        };
+        if let Some(started) = client.handoff_started.take() {
+            client.handoff_wall.add(now.saturating_sub(started));
+            client.handoff_animation.add(elapsed);
+        }
+    }
+
+    pub(crate) fn dump(&self) {
+        // Enabling the feature explicitly opts into the exit report. This does
+        // not depend on a tracing filter and adds no per-frame log messages.
+        eprintln!("resize-metrics: histogram upper bounds ms {BUCKETS_MS:?}, final bucket overflow");
+        eprintln!(
+            "resize-metrics: workspace barrier durations {:?}",
+            self.workspace_duration
+        );
+        for (id, client) in &self.clients {
+            eprintln!("resize-metrics: client {id:?} {client:?}");
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn overlapping_clients_deadlines_and_handoff_are_measured_separately() {
+        let mut metrics = ResizeMetrics::default();
+        let ws = Some(WorkspaceId(1));
+        let ms = Duration::from_millis;
+        metrics.begin(WindowId(1), ws, "terminal", ms(0));
+        metrics.begin(WindowId(2), ws, "editor", ms(50));
+        metrics.observe(WindowId(1), ms(100), true, true);
+        metrics.observe(WindowId(1), ms(300), true, true);
+        metrics.end(WindowId(1), ms(300), End::Deadline);
+        assert!(metrics.workspace_started.contains_key(&WorkspaceId(1)));
+        metrics.end(WindowId(2), ms(310), End::Commit);
+        assert!(metrics.workspace_started.is_empty());
+        metrics.handoff_tick(WindowId(1), ms(310), false);
+        metrics.handoff_end(WindowId(1), ms(390), ms(80));
+        let client = &metrics.clients[&WindowId(1)];
+        assert_eq!(client.deadlines, 1);
+        assert_eq!(client.viewport_pauses, 1);
+        assert_eq!(client.other_window_pauses, 1);
+        assert_eq!(client.viewport_overlap, ms(300));
+        assert_eq!(client.other_overlap, ms(300));
+        assert_eq!(client.barrier_duration.counts[11], 1);
+        assert_eq!(client.handoff_animation.counts[4], 1);
+        assert_eq!(client.handoff_wall.counts[4], 1);
+        assert_eq!(metrics.workspace_duration.counts[13], 1);
+    }
+
+    #[test]
+    fn globally_paused_time_is_not_counted_as_animation_blocking_overlap() {
+        let mut metrics = ResizeMetrics::default();
+        let ms = Duration::from_millis;
+        metrics.begin(WindowId(1), Some(WorkspaceId(1)), "terminal", ms(0));
+        metrics.observe(WindowId(1), ms(40), true, true);
+        metrics.pause_clock(ms(340));
+        metrics.observe(WindowId(1), ms(356), true, true);
+        metrics.end(WindowId(1), ms(356), End::Deadline);
+        let client = &metrics.clients[&WindowId(1)];
+        assert_eq!(client.viewport_overlap, ms(56));
+        assert_eq!(client.other_overlap, ms(56));
+        assert_eq!(client.barrier_duration.counts[15], 1);
+        assert_eq!(client.deadlines, 1);
+    }
+}

compositor/src/state.rs

@@ -17,7 +17,7 @@ use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use calloop::LoopHandle;
-use ferese_animation::{AnimatedValue, ClientSize, PresentationMode, SpringConfig, WindowGeometry};
+use ferese_animation::{AnimatedValue, ClientSize, CrossingPolicy, PresentationMode, SpringConfig, WindowGeometry};
 use ferese_core::{
     LayoutMode, OutputGeometry, OutputId, OutputWorkspaceMap, WindowPlacement, WorkspaceId, WorkspaceLayout,
     WorkspaceSet,
@@ -348,6 +348,9 @@ pub struct Ferese {
     pub(crate) logout_owner: Option<ObjectId>,
     pub(crate) backdrop_generation: u64,
     viewport_animations: HashMap<WorkspaceId, AnimatedValue>,
+    paused_workspaces: HashSet<WorkspaceId>,
+    #[cfg(feature = "resize-metrics")]
+    pub(crate) resize_metrics: crate::resize_metrics::ResizeMetrics,
     focus_swipe: Option<FocusSwipe>,
     workspace_slides: HashMap<OutputId, WorkspaceSlide>,
     workspace_slide_offsets: HashMap<WorkspaceId, (f64, f64)>,
@@ -574,6 +577,9 @@ impl Ferese {
             logout_owner: None,
             backdrop_generation: 0,
             viewport_animations: HashMap::new(),
+            paused_workspaces: HashSet::new(),
+            #[cfg(feature = "resize-metrics")]
+            resize_metrics: crate::resize_metrics::ResizeMetrics::default(),
             focus_swipe: None,
             workspace_slides: HashMap::new(),
             workspace_slide_offsets: HashMap::new(),

compositor/src/state/animation.rs

@@ -3,6 +3,17 @@ use super::*;
 impl Ferese {
     pub fn advance_animations(&mut self, now: Instant) -> bool {
         let delta = crate::presentation::frame_delta(&mut self.last_animation_tick, now);
+        // Consume wall time even while presentation is paused. A paused
+        // desktop must not turn suspend/DPMS time into a spring step.
+        if self.space.outputs().next().is_none()
+            || self.session_lock.sleeping
+            || self.direct_backend.as_ref().is_some_and(|backend| !backend.active)
+        {
+            #[cfg(feature = "resize-metrics")]
+            self.resize_metrics.pause_clock(self.start_time.elapsed());
+            return self.animations_need_tick();
+        }
+
         self.advance_animations_by(delta)
     }
 
@@ -12,16 +23,68 @@ impl Ferese {
         }
     }
 
+    #[cfg(feature = "resize-metrics")]
+    pub(super) fn measure_resize_pauses(&mut self, now: Duration) {
+        if self.session_lock.sleeping || self.direct_backend.as_ref().is_some_and(|backend| !backend.active) {
+            self.resize_metrics.pause_clock(now);
+            return;
+        }
+
+        for id in self.windows.resizing() {
+            let Some(workspace) = self.workspaces.workspace_for_window(*id) else {
+                continue;
+            };
+            let moving = |value: &AnimatedValue| value.current != value.target || value.velocity != 0.0;
+            let viewport = self.viewport_animations.get(&workspace).is_some_and(moving)
+                && self
+                    .output_workspaces
+                    .output_for_workspace(workspace)
+                    .is_some_and(|output| self.output_workspaces.active_workspace(output) == Some(workspace))
+                && self
+                    .focus_swipe
+                    .as_ref()
+                    .is_none_or(|swipe| swipe.workspace != workspace);
+            let others = self.workspaces.workspace(workspace).is_some_and(|workspace| {
+                workspace
+                    .layout
+                    .window_ids()
+                    .chain(workspace.floating.iter().copied())
+                    .any(|other| {
+                        other != *id
+                            && self
+                                .windows
+                                .window(other)
+                                .is_some_and(|window| self.space.element_location(window).is_some())
+                            && self.windows.record(other).is_some_and(|record| {
+                                record.geometry.is_some_and(|mut geometry| {
+                                    geometry.advance(Duration::ZERO, self.spring_config, self.animations_enabled)
+                                }) || record.world_x.as_ref().is_some_and(|(_, value)| moving(value))
+                                    || record.coupled_width.as_ref().is_some_and(|(_, value)| moving(value))
+                            })
+                    })
+            });
+            self.resize_metrics.observe(
+                *id,
+                now,
+                self.animations_enabled && viewport,
+                self.animations_enabled && others,
+            );
+        }
+    }
+
     pub(crate) fn animation_fallback_deadline(&self, interval: std::time::Duration) -> Instant {
         self.last_animation_tick + interval.saturating_mul(2)
     }
 
     pub(crate) fn reset_animation_clock(&mut self) {
         self.last_animation_tick = Instant::now();
+        #[cfg(feature = "resize-metrics")]
+        self.resize_metrics.pause_clock(self.start_time.elapsed());
     }
 
     fn animations_need_tick(&self) -> bool {
         if self.focus_swipe.is_some()
+            || !self.paused_workspaces.is_empty()
             || !self.workspace_slides.is_empty()
             || !self.dismissing_popups.is_empty()
             || self.render.snapshots().next().is_some()
@@ -65,6 +128,11 @@ impl Ferese {
     }
 
     pub(super) fn advance_animations_by(&mut self, delta: std::time::Duration) -> bool {
+        self.advance_animations_at(delta, self.start_time.elapsed())
+    }
+
+    // The production caller supplies monotonic wall time; tests supply a fake clock.
+    fn advance_animations_at(&mut self, delta: Duration, now: Duration) -> bool {
         if !self.animations_need_tick() {
             self.sync_window_stacking();
             return false;
@@ -168,7 +236,23 @@ impl Ferese {
             self.send_window_close(id);
         }
 
-        let now = self.start_time.elapsed();
+        // Include transactions expiring on this dispatch: their waiting time
+        // must not be charged to the first resumed animation step.
+        let paused_before_expiry = self
+            .windows
+            .resizing()
+            .filter_map(|id| self.workspaces.workspace_for_window(*id))
+            .collect::<HashSet<_>>();
+        #[cfg(feature = "resize-metrics")]
+        self.measure_resize_pauses(now);
+        #[cfg(feature = "resize-metrics")]
+        {
+            for (&id, record) in self.windows.records() {
+                if record.resize.is_some_and(|transaction| transaction.expired(now)) {
+                    self.resize_metrics.end(id, now, crate::resize_metrics::End::Deadline);
+                }
+            }
+        }
         self.windows.expire_transactions(now);
 
         let blocked_workspaces = self
@@ -176,8 +260,12 @@ impl Ferese {
             .resizing()
             .filter_map(|id| self.workspaces.workspace_for_window(*id))
             .collect::<HashSet<_>>();
+        let mut resume_pauses = std::mem::replace(&mut self.paused_workspaces, blocked_workspaces.clone());
+        resume_pauses.extend(paused_before_expiry);
         let animations_enabled = self.animations_enabled;
         let animation_speed = self.animation_speed;
+        #[cfg(feature = "resize-metrics")]
+        let resize_metrics = &mut self.resize_metrics;
         self.render.retain_snapshots(|id, snapshot| {
             if !animations_enabled {
                 return false;
@@ -201,6 +289,8 @@ impl Ferese {
                 })
             });
             let blocked = waiting_for_client || uncovered;
+            #[cfg(feature = "resize-metrics")]
+            resize_metrics.handoff_tick(*id, now, blocked);
             let active = crate::presentation::advance_handoff(
                 &mut snapshot.elapsed,
                 &mut snapshot.last_tick,
@@ -213,13 +303,15 @@ impl Ferese {
             }
 
             if !active {
+                #[cfg(feature = "resize-metrics")]
+                resize_metrics.handoff_end(*id, now, snapshot.elapsed);
                 tracing::debug!(?id, bytes = snapshot.bytes(), "released resize handoff snapshot");
             }
             active
         });
         active_animation |= self.render.snapshots().next().is_some();
         // Keep scheduling frames while waiting, so the deadline cannot stall.
-        active_animation |= !blocked_workspaces.is_empty();
+        active_animation |= !blocked_workspaces.is_empty() || !resume_pauses.is_empty();
         for (workspace, viewport) in &mut self.viewport_animations {
             if !self
                 .output_workspaces
@@ -229,7 +321,7 @@ impl Ferese {
                 continue;
             }
 
-            if blocked_workspaces.contains(workspace) {
+            if blocked_workspaces.contains(workspace) || resume_pauses.contains(workspace) {
                 continue;
             }
 
@@ -237,7 +329,8 @@ impl Ferese {
                 viewport.current = swipe.position();
                 viewport.velocity = 0.0;
             } else if self.animations_enabled {
-                active_animation |= viewport.advance(delta, self.viewport_spring_config);
+                active_animation |=
+                    viewport.advance_with_policy(delta, self.viewport_spring_config, CrossingPolicy::NoCrossing);
             } else {
                 viewport.snap();
             }
@@ -253,7 +346,7 @@ impl Ferese {
             if self
                 .workspaces
                 .workspace_for_window(id)
-                .is_some_and(|workspace| blocked_workspaces.contains(&workspace))
+                .is_some_and(|workspace| blocked_workspaces.contains(&workspace) || resume_pauses.contains(&workspace))
             {
                 continue;
             }
@@ -288,23 +381,22 @@ impl Ferese {
                 && let Some(viewport) = self.viewport_animations.get(workspace)
             {
                 if zooming {
-                    world_x.current = geometry.visual.current.x + viewport.current;
-                    world_x.velocity = geometry.visual.velocity.x + viewport.velocity;
+                    sync_scrolling_coordinates(geometry, world_x, viewport, true);
                 } else if self.animations_enabled {
-                    active_animation |= world_x.advance(delta, self.spring_config);
+                    active_animation |=
+                        world_x.advance_with_policy(delta, self.spring_config, CrossingPolicy::NoCrossing);
                 } else {
                     world_x.snap();
                 }
 
                 if !zooming {
-                    geometry.visual.current.x = world_x.current - viewport.current;
-                    geometry.visual.velocity.x = world_x.velocity - viewport.velocity;
+                    sync_scrolling_coordinates(geometry, world_x, viewport, false);
                 }
             }
 
             if let Some((_, width)) = record.coupled_width.as_mut() {
                 let width_active = if self.animations_enabled {
-                    width.advance(delta, self.viewport_spring_config)
+                    width.advance_with_policy(delta, self.viewport_spring_config, CrossingPolicy::NoCrossing)
                 } else {
                     width.snap();
                     false
@@ -461,6 +553,25 @@ impl Ferese {
 
 // Inspect current targets, not the previous frame's activity. Target changes
 // and client waits must wake a desktop that was settled on its last tick.
+/// Zoom owns the visual rectangle until its final handoff tick. Otherwise the
+/// strip coordinates own x. Keep this ownership decision explicit at callers.
+pub(super) fn sync_scrolling_coordinates(
+    geometry: &mut WindowGeometry,
+    world: &mut AnimatedValue,
+    viewport: &AnimatedValue,
+    zoom_owns: bool,
+) {
+    if zoom_owns {
+        world.current = geometry.visual.current.x + viewport.current;
+        world.velocity = geometry.visual.velocity.x + viewport.velocity;
+        debug_assert!((world.current - geometry.visual.current.x - viewport.current).abs() < 1e-6);
+    } else {
+        geometry.visual.current.x = world.current - viewport.current;
+        geometry.visual.velocity.x = world.velocity - viewport.velocity;
+        debug_assert!((geometry.logical.x - world.target + viewport.target).abs() < 1e-6);
+    }
+}
+
 fn record_needs_tick(
     record: &super::window_registry::WindowRecord,
     focus: f64,
@@ -564,4 +675,217 @@ mod tests {
         record.coupled_width = Some((WorkspaceId(1), AnimatedValue::new(400.0)));
         assert!(record_needs_tick(&record, 1.0, 0.0, || true));
     }
+
+    #[test]
+    fn scrolling_target_changes_and_zoom_ownership_switches_are_continuous() {
+        let config = SpringConfig::default();
+        let mut viewport = AnimatedValue::new(100.0);
+        viewport.set_target(400.0);
+        viewport.advance_with_policy(Duration::from_millis(40), config, CrossingPolicy::NoCrossing);
+        let mut world = AnimatedValue::new(500.0);
+        world.set_target(900.0);
+        world.advance_with_policy(Duration::from_millis(40), config, CrossingPolicy::NoCrossing);
+        let mut width = AnimatedValue::new(400.0);
+        width.set_target(700.0);
+        width.advance_with_policy(Duration::from_millis(40), config, CrossingPolicy::NoCrossing);
+        let mut geometry = WindowGeometry::new(
+            Rect::new(world.current - viewport.current, 0.0, width.current, 300.0),
+            None,
+        );
+        let target = Rect::new(world.target - viewport.target, 0.0, width.target, 300.0);
+        geometry.set_logical_target(target, Duration::from_millis(40));
+        sync_scrolling_coordinates(&mut geometry, &mut world, &viewport, false);
+        geometry.visual.velocity.width = width.velocity;
+        let before = geometry.visual.current;
+        viewport.retarget_preserving_motion(300.0);
+        world.retarget_preserving_motion(1000.0);
+        width.retarget_preserving_motion(600.0);
+        geometry.set_logical_target(
+            Rect::new(world.target - viewport.target, 0.0, width.target, 300.0),
+            Duration::from_millis(40),
+        );
+        sync_scrolling_coordinates(&mut geometry, &mut world, &viewport, false);
+        assert_eq!(geometry.visual.current, before);
+        assert_eq!(geometry.visual.velocity.x, world.velocity - viewport.velocity);
+        assert_eq!(geometry.logical.x, world.target - viewport.target);
+
+        for mode in [
+            PresentationMode::Maximized,
+            PresentationMode::Fullscreen,
+            PresentationMode::Normal,
+        ] {
+            let before = geometry.visual.current;
+            geometry.set_presentation_mode(Rect::new(0.0, 0.0, 1920.0, 1080.0), mode, Duration::from_millis(40));
+            sync_scrolling_coordinates(&mut geometry, &mut world, &viewport, true);
+            assert_eq!(geometry.visual.current, before);
+            assert_eq!(world.current, geometry.visual.current.x + viewport.current);
+            geometry.advance(Duration::ZERO, config, true);
+            assert_eq!(geometry.visual.current, before);
+            geometry.advance(Duration::from_millis(16), config, true);
+        }
+
+        // Hand back to scrolling after a completed zoom. world_x was mirrored
+        // on the final zoom tick; changing its destination must not change x.
+        for _ in 0..200 {
+            let zoom_owned = geometry.is_zooming();
+            geometry.advance(Duration::from_millis(16), config, true);
+            if zoom_owned {
+                sync_scrolling_coordinates(&mut geometry, &mut world, &viewport, true);
+            }
+            if !geometry.is_zooming() {
+                break;
+            }
+        }
+        assert!(!geometry.is_zooming());
+        let before = geometry.visual.current;
+        world.set_target(geometry.logical.x + viewport.target);
+        sync_scrolling_coordinates(&mut geometry, &mut world, &viewport, false);
+        assert_eq!(geometry.visual.current, before);
+    }
+
+    #[test]
+    #[ignore = "requires a private XDG_RUNTIME_DIR and permission to bind test sockets"]
+    fn resize_pause_and_resume_use_fake_wall_time_without_charging_the_pause() {
+        if std::env::var_os("FERESE_ANIMATION_PAUSE_TEST_CHILD").is_none() {
+            let runtime = tempfile::tempdir().unwrap();
+            let output = std::process::Command::new(std::env::current_exe().unwrap())
+                .args([
+                    "--exact",
+                    "state::animation::tests::resize_pause_and_resume_use_fake_wall_time_without_charging_the_pause",
+                    "--ignored",
+                    "--nocapture",
+                ])
+                .env("FERESE_ANIMATION_PAUSE_TEST_CHILD", "1")
+                .env("XDG_RUNTIME_DIR", runtime.path())
+                .env_remove("FERESE_SOCKET")
+                .output()
+                .unwrap();
+            assert!(
+                output.status.success(),
+                "{}\n{}",
+                String::from_utf8_lossy(&output.stdout),
+                String::from_utf8_lossy(&output.stderr)
+            );
+            return;
+        }
+
+        let runtime = std::path::PathBuf::from(std::env::var_os("XDG_RUNTIME_DIR").unwrap());
+        assert!(runtime.starts_with(std::env::temp_dir()));
+        let mut event_loop = EventLoop::try_new().unwrap();
+        let display = Display::new().unwrap();
+        let config = crate::config::Config::default().runtime_config().unwrap();
+        let mut state = Ferese::new(&mut event_loop, display, config).unwrap();
+        let output = Output::new(
+            "pause-test".into(),
+            smithay::output::PhysicalProperties {
+                size: (0, 0).into(),
+                subpixel: smithay::output::Subpixel::Unknown,
+                make: "test".into(),
+                model: "test".into(),
+            },
+        );
+        output.change_current_state(
+            Some(smithay::output::Mode {
+                size: (1920, 1080).into(),
+                refresh: 60000,
+            }),
+            None,
+            None,
+            None,
+        );
+        state.space.map_output(&output, (0, 0));
+        state.register_output(&output, "pause-test".into());
+        let workspace = state.workspaces.active_id();
+        let id = WindowId(1);
+        state.workspaces.insert_window(id, Axis::Horizontal, 0.5).unwrap();
+        state.windows.records.insert(id, settled());
+        let mut viewport = AnimatedValue::new(0.0);
+        viewport.set_target(500.0);
+        state.viewport_animations.insert(workspace, viewport);
+        state.advance_animations_at(Duration::from_millis(40), Duration::from_millis(40));
+        let frozen = state.viewport_animations[&workspace];
+        assert!(frozen.current > 0.0 && frozen.current < 500.0);
+
+        // No intervening dispatch at all: expiration happens on the 300ms tick.
+        state.windows.set_transaction(
+            id,
+            crate::resize_transaction::ResizeTransaction::new(1.into(), Duration::from_millis(40)),
+        );
+        state.advance_animations_at(Duration::from_millis(300), Duration::from_millis(340));
+        assert!(state.windows.transaction(&id).is_none());
+        assert_eq!(state.viewport_animations[&workspace], frozen);
+        state.advance_animations_at(Duration::from_millis(16), Duration::from_millis(356));
+        let mut expected = frozen;
+        expected.advance_with_policy(
+            Duration::from_millis(16),
+            state.viewport_spring_config,
+            CrossingPolicy::NoCrossing,
+        );
+        assert_eq!(state.viewport_animations[&workspace], expected);
+
+        // A commit releases the barrier between ticks. Drop that release tick's
+        // elapsed pause too, then resume at the next normal 16ms interval.
+        state.windows.set_transaction(
+            id,
+            crate::resize_transaction::ResizeTransaction::new(2.into(), Duration::from_millis(356)),
+        );
+        state.advance_animations_at(Duration::from_millis(100), Duration::from_millis(456));
+        let frozen = state.viewport_animations[&workspace];
+        state.windows.clear_transaction(&id);
+        state.advance_animations_at(Duration::from_millis(200), Duration::from_millis(656));
+        assert_eq!(state.viewport_animations[&workspace], frozen);
+        state.advance_animations_at(Duration::from_millis(16), Duration::from_millis(672));
+        let mut expected = frozen;
+        expected.advance_with_policy(
+            Duration::from_millis(16),
+            state.viewport_spring_config,
+            CrossingPolicy::NoCrossing,
+        );
+        assert_eq!(state.viewport_animations[&workspace], expected);
+
+        let frozen = state.viewport_animations[&workspace];
+        let instant = Instant::now();
+        state.last_animation_tick = instant;
+        state.session_lock.sleeping = true;
+        state.advance_animations(instant + Duration::from_millis(300));
+        assert_eq!(state.viewport_animations[&workspace], frozen);
+        state.session_lock.sleeping = false;
+        state.advance_animations(instant + Duration::from_millis(316));
+        let mut expected = frozen;
+        expected.advance_with_policy(
+            Duration::from_millis(16),
+            state.viewport_spring_config,
+            CrossingPolicy::NoCrossing,
+        );
+        assert_eq!(state.viewport_animations[&workspace], expected);
+
+        // Wake paths reset the global clock even when nothing dispatched while
+        // asleep. This excludes a whole undelivered pause interval as well.
+        let frozen = state.viewport_animations[&workspace];
+        state.last_animation_tick = Instant::now() - Duration::from_secs(5);
+        state.reset_animation_clock();
+        let resumed = state.last_animation_tick;
+        state.advance_animations(resumed + Duration::from_millis(16));
+        let mut expected = frozen;
+        expected.advance_with_policy(
+            Duration::from_millis(16),
+            state.viewport_spring_config,
+            CrossingPolicy::NoCrossing,
+        );
+        assert_eq!(state.viewport_animations[&workspace], expected);
+
+        // A settled transaction still has to flush its resume guard; otherwise
+        // the next unrelated animation would inherit an old paused workspace.
+        state.viewport_animations.get_mut(&workspace).unwrap().snap();
+        state.windows.record_mut(id).unwrap().focus = Some(DimAnimation::new(0.0));
+        state.windows.set_transaction(
+            id,
+            crate::resize_transaction::ResizeTransaction::new(3.into(), Duration::from_millis(800)),
+        );
+        state.advance_animations_at(Duration::from_millis(100), Duration::from_millis(900));
+        assert!(state.paused_workspaces.contains(&workspace));
+        state.windows.clear_transaction(&id);
+        state.advance_animations_at(Duration::from_millis(16), Duration::from_millis(916));
+        assert!(state.paused_workspaces.is_empty());
+    }
 }

compositor/src/state/floating.rs

@@ -164,6 +164,9 @@ impl Ferese {
 
         // Direct manipulation follows the pointer, including while the client is
         // still drawing its next buffer. Rendering and hit testing share this rect.
+        #[cfg(feature = "resize-metrics")]
+        self.resize_metrics
+            .end(id, self.start_time.elapsed(), crate::resize_metrics::End::Cancelled);
         self.windows.clear_transaction(&id);
         self.render.clear_snapshot(&id);
         let Some(record) = self.windows.record_mut(id) else {

compositor/src/state/layout.rs

@@ -261,15 +261,27 @@ impl Ferese {
                     .remove(&id)
                     .filter(|(previous_workspace, _)| *previous_workspace == workspace)
                     .map(|(_, world_x)| world_x)
-                    .unwrap_or_else(|| AnimatedValue::new(restored_world_x));
+                    .unwrap_or_else(|| {
+                        let mut world = AnimatedValue::new(restored_world_x);
+                        world.velocity = geometry.visual.velocity.x
+                            + self
+                                .viewport_animations
+                                .get(&workspace)
+                                .map_or(0.0, |viewport| viewport.velocity);
+                        world
+                    });
                 animated_world_x.set_target(world_x);
                 if !self.animations_enabled {
                     animated_world_x.snap();
                 }
 
-                if !geometry.is_zooming() {
-                    geometry.visual.current.x = animated_world_x.current - viewport_x;
-                    geometry.visual.velocity.x = animated_world_x.velocity;
+                if let Some(viewport) = self.viewport_animations.get(&workspace) {
+                    super::animation::sync_scrolling_coordinates(
+                        geometry,
+                        &mut animated_world_x,
+                        viewport,
+                        geometry.is_zooming(),
+                    );
                 }
                 record.world_x = Some((workspace, animated_world_x));
 
@@ -347,6 +359,14 @@ impl Ferese {
                     && had_geometry
                     && self.animations_enabled
                 {
+                    self.paused_workspaces.extend(self.workspaces.workspace_for_window(id));
+                    #[cfg(feature = "resize-metrics")]
+                    self.resize_metrics.begin(
+                        id,
+                        self.workspaces.workspace_for_window(id),
+                        &self.windows.record(id).unwrap().app_id,
+                        now,
+                    );
                     self.windows.set_transaction(
                         id,
                         crate::resize_transaction::ResizeTransaction::new(serial, now)

compositor/src/state/outputs.rs

@@ -2,6 +2,10 @@ use super::*;
 
 impl Ferese {
     pub fn register_output(&mut self, output: &Output, identity: String) {
+        if self.output_ids.is_empty() {
+            self.reset_animation_clock();
+        }
+
         if self.session_lock.active {
             self.session_lock.output_added(output);
             self.lock_input_activity();

compositor/src/state/prediction.rs

@@ -89,10 +89,15 @@ impl Ferese {
                     .focus_swipe
                     .as_ref()
                     .is_some_and(|swipe| swipe.workspace == workspace);
-                !held && viewport.advance(Duration::ZERO, self.viewport_spring_config)
+                !held
+                    && viewport.advance_with_policy(
+                        Duration::ZERO,
+                        self.viewport_spring_config,
+                        CrossingPolicy::NoCrossing,
+                    )
             }) || record.coupled_width.as_ref().is_some_and(|(_, width)| {
                 let mut width = *width;
-                width.advance(Duration::ZERO, self.viewport_spring_config)
+                width.advance_with_policy(Duration::ZERO, self.viewport_spring_config, CrossingPolicy::NoCrossing)
             })
         })
     }
@@ -100,7 +105,7 @@ impl Ferese {
     pub(crate) fn sample_frame(&self, output: &Output, horizon: Duration) -> FrameScene {
         let animating = self.output_has_animations(output);
         let delta = if self.animations_enabled && animating {
-            horizon.min(Duration::from_millis(100)).mul_f64(self.animation_speed)
+            horizon.mul_f64(self.animation_speed)
         } else {
             Duration::ZERO
         };
@@ -219,9 +224,9 @@ fn predict_geometry(
     if let Some((mut world, mut viewport, held)) = world
         && !zooming
     {
-        world.advance(delta, spring);
+        world.advance_with_policy(delta, spring, CrossingPolicy::NoCrossing);
         if !held {
-            viewport.advance(delta, viewport_spring);
+            viewport.advance_with_policy(delta, viewport_spring, CrossingPolicy::NoCrossing);
         }
 
         predicted.visual.current.x = world.current - viewport.current;
@@ -229,7 +234,7 @@ fn predict_geometry(
     }
 
     if let Some(mut width) = width {
-        width.advance(delta, viewport_spring);
+        width.advance_with_policy(delta, viewport_spring, CrossingPolicy::NoCrossing);
         predicted.visual.current.width = width.current;
         predicted.visual.velocity.width = width.velocity;
     }
@@ -463,9 +468,9 @@ mod tests {
             Some((world, viewport, false)),
             Some(width),
         );
-        world.advance(delta, spring);
-        viewport.advance(delta, spring);
-        width.advance(delta, spring);
+        world.advance_with_policy(delta, spring, CrossingPolicy::NoCrossing);
+        viewport.advance_with_policy(delta, spring, CrossingPolicy::NoCrossing);
+        width.advance_with_policy(delta, spring, CrossingPolicy::NoCrossing);
         assert_eq!(predicted.visual.current.x, world.current - viewport.current);
         assert_eq!(predicted.visual.current.width, width.current);
         assert_eq!(predicted.visual.target, geometry.visual.target);

compositor/src/state/windows.rs

@@ -424,6 +424,9 @@ impl Ferese {
         };
 
         self.focus_history.remove(id);
+        #[cfg(feature = "resize-metrics")]
+        self.resize_metrics
+            .end(id, self.start_time.elapsed(), crate::resize_metrics::End::Cancelled);
         self.render.remove_window(id);
         self.window_stack.remove(id);
         self.floating_above_fullscreen.remove(&id);
@@ -541,6 +544,12 @@ impl Ferese {
             .transaction(&id)
             .is_some_and(|transaction| transaction.accepts(committed_serial))
         {
+            #[cfg(feature = "resize-metrics")]
+            {
+                self.measure_resize_pauses(self.start_time.elapsed());
+                self.resize_metrics
+                    .end(id, self.start_time.elapsed(), crate::resize_metrics::End::Commit);
+            }
             self.windows.clear_transaction(&id);
         }
 
@@ -578,6 +587,9 @@ impl Ferese {
                 // This is the first mapped client buffer, not a user resize.
                 // Do not animate from the temporary placement box.
                 self.windows.set_geometry(id, WindowGeometry::new(rect, Some(size)));
+                #[cfg(feature = "resize-metrics")]
+                self.resize_metrics
+                    .end(id, self.start_time.elapsed(), crate::resize_metrics::End::Cancelled);
                 self.windows.clear_transaction(&id);
                 self.relayout();
             }
@@ -614,6 +626,9 @@ impl Ferese {
 
             if self.workspaces.set_floating_rect(id, rect).is_ok() {
                 self.windows.set_geometry(id, WindowGeometry::new(rect, Some(size)));
+                #[cfg(feature = "resize-metrics")]
+                self.resize_metrics
+                    .end(id, self.start_time.elapsed(), crate::resize_metrics::End::Cancelled);
                 self.windows.clear_transaction(&id);
                 self.render.clear_snapshot(&id);
                 self.relayout();

crates/ferese-animation/src/lib.rs

@@ -2,8 +2,15 @@ use std::time::Duration;
 
 use ferese_layout::Rect;
 
-const MAX_FRAME_DELTA: Duration = Duration::from_millis(100);
-const MAX_STEP_SECONDS: f64 = 1.0 / 240.0;
+mod spring;
+
+/// Crossing behavior belongs to the property, not the physical solver.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum CrossingPolicy {
+    NoCrossing,
+    AllowOvershoot,
+}
+
 const CLIENT_COMMIT_TIMEOUT: Duration = Duration::from_millis(500);
 
 #[derive(Clone, Copy, Debug, PartialEq)]
@@ -93,32 +100,35 @@ impl AnimatedValue {
         self.velocity = 0.0;
     }
 
+    /// Compatibility entry point; existing presentation properties do not cross.
     pub fn advance(&mut self, delta: Duration, config: SpringConfig) -> bool {
+        self.advance_with_policy(delta, config, CrossingPolicy::NoCrossing)
+    }
+
+    pub fn advance_with_policy(&mut self, delta: Duration, config: SpringConfig, policy: CrossingPolicy) -> bool {
         let config = config.normalized();
-        if (self.current - self.target).abs() <= config.position_tolerance
-            && self.velocity.abs() <= config.velocity_tolerance
-        {
+        let settled = |value: &Self| {
+            (value.current - value.target).abs() <= config.position_tolerance
+                && value.velocity.abs() <= config.velocity_tolerance
+        };
+        if delta.is_zero() {
+            return !settled(self);
+        }
+
+        if settled(self) {
             self.snap();
             return false;
         }
 
-        let seconds = delta.min(MAX_FRAME_DELTA).as_secs_f64();
-        let steps = (seconds / MAX_STEP_SECONDS).ceil().max(1.0) as usize;
-        let step = seconds / steps as f64;
-
-        for _ in 0..steps {
-            let previous_error = self.current - self.target;
-            advance_value(&mut self.current, self.target, &mut self.velocity, step, config);
-            let current_error = self.current - self.target;
-            if previous_error != 0.0 && previous_error.signum() != current_error.signum() {
-                self.snap();
-                break;
-            }
-        }
-
-        if (self.current - self.target).abs() <= config.position_tolerance
-            && self.velocity.abs() <= config.velocity_tolerance
-        {
+        advance_property(
+            &mut self.current,
+            self.target,
+            &mut self.velocity,
+            delta.as_secs_f64(),
+            config,
+            policy,
+        );
+        if settled(self) {
             self.snap();
             false
         } else {
@@ -146,34 +156,54 @@ impl AnimatedRect {
     }
 
     pub fn advance(&mut self, delta: Duration, config: SpringConfig) -> bool {
+        self.advance_with_policy(delta, config, CrossingPolicy::NoCrossing)
+    }
+
+    pub fn advance_with_policy(&mut self, delta: Duration, config: SpringConfig, policy: CrossingPolicy) -> bool {
+        let config = config.normalized();
+        if delta.is_zero() {
+            return !self.is_settled(config);
+        }
+
         if self.is_settled(config) {
             self.snap();
             return false;
         }
 
-        let config = config.normalized();
-        let seconds = delta.min(MAX_FRAME_DELTA).as_secs_f64();
-        let steps = (seconds / MAX_STEP_SECONDS).ceil().max(1.0) as usize;
-        let step = seconds / steps as f64;
-
-        for _ in 0..steps {
-            advance_value(&mut self.current.x, self.target.x, &mut self.velocity.x, step, config);
-            advance_value(&mut self.current.y, self.target.y, &mut self.velocity.y, step, config);
-            advance_value(
+        // The old crossing clamp applied independently to each component.
+        let dt = delta.as_secs_f64();
+        advance_property(
+            &mut self.current.x,
+            self.target.x,
+            &mut self.velocity.x,
+            dt,
+            config,
+            policy,
+        );
+        advance_property(
+            &mut self.current.y,
+            self.target.y,
+            &mut self.velocity.y,
+            dt,
+            config,
+            policy,
+        );
+        advance_property(
             &mut self.current.width,
             self.target.width,
             &mut self.velocity.width,
-                step,
+            dt,
             config,
+            policy,
         );
-            advance_value(
+        advance_property(
             &mut self.current.height,
             self.target.height,
             &mut self.velocity.height,
-                step,
+            dt,
             config,
+            policy,
         );
-        }
 
         if self.is_settled(config) {
             self.snap();
@@ -300,9 +330,15 @@ impl WindowGeometry {
 
     pub fn set_logical_target(&mut self, rect: Rect, now: Duration) -> Option<ClientSize> {
         let rect = normalized_rect(rect);
+        if self.zoom.is_some() {
+            return self.set_presentation_mode(rect, self.mode, now);
+        }
+
+        let before = self.visual.current;
         let size = ClientSize::from_rect(rect);
         self.logical = rect;
         self.visual.set_target(rect);
+        debug_assert_eq!(self.visual.current, before);
 
         self.client.request_size(size, now).then_some(size)
     }
@@ -321,6 +357,7 @@ impl WindowGeometry {
 
     pub fn set_presentation_mode(&mut self, rect: Rect, mode: PresentationMode, now: Duration) -> Option<ClientSize> {
         let rect = normalized_rect(rect);
+        let before = (self.visual.current, self.decorations);
         if self.mode != mode || (self.zoom.is_some() && self.logical != rect) {
             let mut progress = AnimatedValue::new(0.0);
             progress.set_target(1.0);
@@ -348,6 +385,7 @@ impl WindowGeometry {
         self.mode = mode;
         self.logical = rect;
         self.visual.set_target(rect);
+        debug_assert_eq!((self.visual.current, self.decorations), before);
         self.presentation_size_request(now)
     }
 
@@ -398,6 +436,10 @@ impl WindowGeometry {
             return false;
         }
 
+        if delta.is_zero() {
+            return self.zoom.is_some() || !self.visual.is_settled(config);
+        }
+
         if let Some(zoom) = &mut self.zoom {
             let target = self.visual.target;
             // Spring tolerances describe pixels and pixels/second, whereas this
@@ -413,7 +455,9 @@ impl WindowGeometry {
                 velocity_tolerance: config.velocity_tolerance / travel,
                 ..config
             };
-            let active = zoom.progress.advance(delta, progress_config);
+            let active = zoom
+                .progress
+                .advance_with_policy(delta, progress_config, CrossingPolicy::NoCrossing);
             let p = zoom.progress.current;
             let lerp = |a: f64, b: f64| a + (b - a) * p;
             self.visual.current = Rect::new(
@@ -443,7 +487,8 @@ impl WindowGeometry {
             return active;
         }
 
-        self.visual.advance(delta, config)
+        self.visual
+            .advance_with_policy(delta, config, CrossingPolicy::NoCrossing)
     }
 
     pub fn inverse_visual_point(&self, x: f64, y: f64) -> Option<(f64, f64)> {
@@ -467,19 +512,27 @@ impl WindowGeometry {
     }
 }
 
-fn advance_value(current: &mut f64, target: f64, velocity: &mut f64, delta: f64, config: SpringConfig) {
-    let acceleration = (-config.stiffness * (*current - target) - config.damping * *velocity) / config.mass;
-    *velocity += acceleration * delta;
-    let previous_error = *current - target;
-    *current += *velocity * delta;
-    let current_error = *current - target;
-
-    if previous_error != 0.0 && previous_error.signum() != current_error.signum() {
+fn advance_property(
+    current: &mut f64,
+    target: f64,
+    velocity: &mut f64,
+    dt: f64,
+    config: SpringConfig,
+    policy: CrossingPolicy,
+) {
+    if policy == CrossingPolicy::NoCrossing && spring::crosses(*current, *velocity, target, dt, config) {
         *current = target;
         *velocity = 0.0;
+    } else {
+        (*current, *velocity) = spring::solve(*current, *velocity, target, dt, config);
     }
 }
 
+#[cfg(test)]
+fn advance_value(current: &mut f64, target: f64, velocity: &mut f64, dt: f64, config: SpringConfig) {
+    (*current, *velocity) = spring::solve(*current, *velocity, target, dt, config);
+}
+
 fn finite_or_zero(value: f64) -> f64 {
     if value.is_finite() { value } else { 0.0 }
 }
@@ -893,7 +946,7 @@ mod tests {
     }
 
     #[test]
-    fn scalar_spring_does_not_overshoot_target() {
+    fn scalar_presentation_clamp_does_not_overshoot_and_clears_velocity() {
         let mut value = AnimatedValue::new(0.0);
         value.velocity = 20_000.0;
         value.set_target(100.0);
@@ -901,6 +954,9 @@ mod tests {
         for _ in 0..60 {
             value.advance(Duration::from_secs_f64(1.0 / 60.0), SpringConfig::default());
             assert!(value.current <= 100.0);
+            if value.current == value.target {
+                assert_eq!(value.velocity, 0.0);
+            }
         }
     }
 
@@ -1024,3 +1080,6 @@ mod tests {
         assert_eq!(geometry.visual.current, target);
     }
 }
+
+#[cfg(test)]
+mod traces;

crates/ferese-animation/src/spring.rs

@@ -0,0 +1,96 @@
+//! Exact physical spring motion. Presentation policy is deliberately separate.
+use super::SpringConfig;
+
+/// Solve m y'' + c y' + k y = 0, with y = current - target.
+/// No settling tolerance or crossing clamp is applied here.
+pub(super) fn solve(current: f64, velocity: f64, target: f64, dt: f64, config: SpringConfig) -> (f64, f64) {
+    if dt <= 0.0 || !dt.is_finite() {
+        return (current, velocity);
+    }
+
+    let config = config.normalized();
+    let omega2 = config.stiffness / config.mass;
+    let omega = omega2.sqrt();
+    let a = config.damping / (2.0 * config.mass);
+    let d = (a - omega) * (a + omega);
+    let y = current - target;
+    let z2 = d * dt * dt;
+
+    // cos/sinc and cosh/sinhc have the same power series in signed z².
+    // This keeps the actual discriminant, including near-critical damping,
+    // rather than replacing a neighborhood with the critical solution.
+    if z2.abs() < 0.01 {
+        let c = 1.0 + z2 * (0.5 + z2 * (1.0 / 24.0 + z2 * (1.0 / 720.0 + z2 / 40320.0)));
+        let s = dt * (1.0 + z2 * (1.0 / 6.0 + z2 * (1.0 / 120.0 + z2 * (1.0 / 5040.0 + z2 / 362880.0))));
+        let e = (-a * dt).exp();
+        return (
+            target + e * (y * c + (velocity + a * y) * s),
+            e * (velocity * c - (a * velocity + omega2 * y) * s),
+        );
+    }
+
+    if d < 0.0 {
+        let w = (-d).sqrt();
+        let (sin, cos) = (w * dt).sin_cos();
+        let e = (-a * dt).exp();
+        let s = sin / w;
+        (
+            target + e * (y * cos + (velocity + a * y) * s),
+            e * (velocity * cos - (a * velocity + omega2 * y) * s),
+        )
+    } else {
+        // Avoid -a + sqrt(a²-omega²) cancellation and exp/cosh overflow.
+        let q = d.sqrt();
+        let fast = -a - q;
+        let slow = -omega2 / (a + q);
+        let slow_coefficient = (velocity - fast * y) / (2.0 * q);
+        let fast_coefficient = (slow * y - velocity) / (2.0 * q);
+        let slow_term = slow_coefficient * (slow * dt).exp();
+        let fast_term = fast_coefficient * (fast * dt).exp();
+        (target + slow_term + fast_term, slow * slow_term + fast * fast_term)
+    }
+}
+
+/// True if the physical trajectory reaches equilibrium anywhere in (0, dt].
+/// Endpoint-only tests miss an even number of crossings after a long stall.
+pub(super) fn crosses(current: f64, velocity: f64, target: f64, dt: f64, config: SpringConfig) -> bool {
+    if dt <= 0.0 {
+        return false;
+    }
+
+    let config = config.normalized();
+    let a = config.damping / (2.0 * config.mass);
+    let omega = (config.stiffness / config.mass).sqrt();
+    let d = (a - omega) * (a + omega);
+    let y = current - target;
+    let b = velocity + a * y;
+    if y == 0.0 && velocity == 0.0 {
+        return false;
+    }
+
+    let crossing = if d < 0.0 {
+        let w = (-d).sqrt();
+        let phase = (-y * w).atan2(b).rem_euclid(std::f64::consts::PI);
+        let phase = if phase == 0.0 { std::f64::consts::PI } else { phase };
+        phase / w
+    } else if y == 0.0 || b == 0.0 {
+        return false;
+    } else {
+        let t = -y / b;
+        if t <= 0.0 {
+            return false;
+        }
+
+        // atanh(q*t)/q = t * (1 + (q*t)²/3 + ...), including q = 0.
+        let z2 = d * t * t;
+        if z2 < 1e-8 {
+            t * (1.0 + z2 * (1.0 / 3.0 + z2 / 5.0))
+        } else if z2 < 1.0 {
+            let q = d.sqrt();
+            (q * t).atanh() / q
+        } else {
+            return false;
+        }
+    };
+    crossing <= dt
+}

crates/ferese-animation/src/traces.rs

@@ -0,0 +1,604 @@
+//! Deterministic elapsed-time traces and the frozen pre-analytic oracle.
+use super::*;
+
+#[derive(Debug, Clone, Copy)]
+struct Sample {
+    time: Duration,
+    current: f64,
+    velocity: f64,
+    target: f64,
+}
+
+struct FakeClock {
+    now: Duration,
+    last: Duration,
+    paused: bool,
+    trace: Vec<Sample>,
+}
+
+impl FakeClock {
+    fn new() -> Self {
+        Self {
+            now: Duration::ZERO,
+            last: Duration::ZERO,
+            paused: false,
+            trace: Vec::new(),
+        }
+    }
+
+    fn sample(&mut self, time: Duration, value: &mut AnimatedValue, config: SpringConfig) -> Sample {
+        assert!(time >= self.now);
+        self.now = time;
+        let delta = time - self.last;
+        self.last = time;
+        if !self.paused {
+            value.advance(delta, config);
+        }
+        let sample = Sample {
+            time,
+            current: value.current,
+            velocity: value.velocity,
+            target: value.target,
+        };
+        self.trace.push(sample);
+        sample
+    }
+
+    fn pause(&mut self) {
+        self.paused = true;
+    }
+
+    fn resume(&mut self, time: Duration) {
+        self.now = time;
+        self.last = time;
+        self.paused = false;
+    }
+}
+
+// Frozen scalar integrator, including its cap, substeps and crossing snap.
+fn legacy_advance(value: &mut AnimatedValue, delta: Duration, config: SpringConfig) {
+    let config = config.normalized();
+    let settled = |v: &AnimatedValue| {
+        (v.current - v.target).abs() <= config.position_tolerance && v.velocity.abs() <= config.velocity_tolerance
+    };
+    if settled(value) {
+        value.snap();
+        return;
+    }
+
+    let seconds = delta.min(Duration::from_millis(100)).as_secs_f64();
+    let steps = (seconds / (1.0 / 240.0)).ceil().max(1.0) as usize;
+    let dt = seconds / steps as f64;
+    for _ in 0..steps {
+        let old = value.current - value.target;
+        value.velocity += (-config.stiffness * old - config.damping * value.velocity) / config.mass * dt;
+        value.current += value.velocity * dt;
+        if old != 0.0 && old.signum() != (value.current - value.target).signum() {
+            value.snap();
+            break;
+        }
+    }
+    if settled(value) {
+        value.snap();
+    }
+}
+
+fn close(a: f64, b: f64, epsilon: f64) {
+    assert!((a - b).abs() <= epsilon, "{a} != {b}, tolerance {epsilon}");
+}
+
+#[test]
+fn capture_old_golden_trace_and_retarget_policy() {
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(500.0);
+    for i in 1..=18 {
+        legacy_advance(&mut value, Duration::from_secs_f64(1.0 / 60.0), SpringConfig::default());
+        if [1, 3, 6, 12, 18].contains(&i) {
+            println!("old normal {i}: {}, {}", value.current, value.velocity);
+        }
+    }
+
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(1000.0);
+    legacy_advance(&mut value, Duration::from_millis(40), SpringConfig::default());
+    println!("old retarget before: {}, {}", value.current, value.velocity);
+    let position = value.current;
+    let velocity = value.velocity;
+    value.retarget_preserving_motion(2000.0);
+    assert_eq!(value.current, position);
+    assert_eq!(value.velocity, velocity);
+    value.retarget_preserving_motion(-500.0);
+    println!("old reversal: {}, {}", value.current, value.velocity);
+    assert_eq!(value.current, position);
+    assert_eq!(value.velocity, velocity * 0.35);
+}
+
+#[test]
+fn long_stall_uses_the_whole_elapsed_duration() {
+    let config = SpringConfig {
+        mass: 1.0,
+        stiffness: 100.0,
+        damping: 20.0,
+        ..SpringConfig::default()
+    };
+    let mut clock = FakeClock::new();
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(500.0);
+    clock.sample(Duration::from_millis(16), &mut value, config);
+    let sample = clock.sample(Duration::from_millis(416), &mut value, config);
+    let t: f64 = 0.416;
+    let decay = (-10.0 * t).exp();
+    close(sample.current, 500.0 - 500.0 * (1.0 + 10.0 * t) * decay, 1e-8);
+    close(sample.velocity, 50000.0 * t * decay, 1e-8);
+    assert_eq!(sample.time, Duration::from_millis(416));
+    assert_eq!(sample.target, 500.0);
+}
+
+#[test]
+fn physical_solver_crosses_equilibrium_without_zeroing_velocity() {
+    let mut x = 0.0;
+    let mut v = 20000.0;
+    advance_value(&mut x, 100.0, &mut v, 0.01, SpringConfig::default());
+    assert!(x > 100.0, "physical solution was snapped: {x}");
+    assert!(v > 0.0, "velocity was erased: {v}");
+}
+
+#[test]
+fn fake_clock_pause_excludes_three_hundred_milliseconds() {
+    let mut clock = FakeClock::new();
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(500.0);
+    clock.sample(Duration::from_millis(40), &mut value, SpringConfig::default());
+    let frozen = value;
+    clock.pause();
+    clock.sample(Duration::from_millis(340), &mut value, SpringConfig::default());
+    assert_eq!(value, frozen);
+    clock.resume(Duration::from_millis(340));
+    clock.sample(Duration::from_millis(356), &mut value, SpringConfig::default());
+    let mut expected = frozen;
+    expected.advance(Duration::from_millis(16), SpringConfig::default());
+    assert_eq!(value, expected);
+}
+
+fn physical(x: f64, v: f64, target: f64, dt: f64, config: SpringConfig) -> (f64, f64) {
+    let (mut x, mut v) = (x, v);
+    advance_value(&mut x, target, &mut v, dt, config);
+    (x, v)
+}
+
+// Independent characteristic-root reference, used away from ill-conditioned roots.
+fn reference(x: f64, v: f64, target: f64, dt: f64, config: SpringConfig) -> (f64, f64) {
+    let a = config.damping / (2.0 * config.mass);
+    let omega2 = config.stiffness / config.mass;
+    let y = x - target;
+    let d = a * a - omega2;
+    if d.abs() < 1e-10 {
+        let b = v + a * y;
+        let e = (-a * dt).exp();
+        return (target + e * (y + b * dt), e * (v - a * b * dt));
+    }
+
+    if d < 0.0 {
+        let w = (-d).sqrt();
+        let b = (v + a * y) / w;
+        let (s, c) = (w * dt).sin_cos();
+        let e = (-a * dt).exp();
+        (target + e * (y * c + b * s), e * (v * c - (a * v + omega2 * y) / w * s))
+    } else {
+        let q = d.sqrt();
+        let r1 = -a + q;
+        let r2 = -a - q;
+        let c1 = (v - r2 * y) / (r1 - r2);
+        let c2 = y - c1;
+        let e1 = (r1 * dt).exp();
+        let e2 = (r2 * dt).exp();
+        (target + c1 * e1 + c2 * e2, c1 * r1 * e1 + c2 * r2 * e2)
+    }
+}
+
+#[test]
+fn solver_reference_regimes_initial_states_and_times() {
+    for damping in [0.0, 5.0, 20.0, 20.0 * (1.0 - 1e-9), 20.0 * (1.0 + 1e-9), 53.0, 2000.0] {
+        let config = SpringConfig {
+            stiffness: 100.0,
+            damping,
+            ..SpringConfig::default()
+        };
+        for (x, v, target) in [
+            (0., 0., 0.),
+            (0., 100., 0.),
+            (500., 0., 0.),
+            (-500., 1234., 200.),
+            (500., -1234., -200.),
+        ] {
+            for dt in [0.0, 1e-12, 0.0083, 0.05, 0.4, 5.0] {
+                let actual = physical(x, v, target, dt, config);
+                let expected = reference(x, v, target, dt, config);
+                close(actual.0, expected.0, 1e-6);
+                close(actual.1, expected.1, 1e-5);
+            }
+        }
+    }
+}
+
+#[test]
+fn near_critical_sweep_has_no_regime_jump() {
+    let base = SpringConfig {
+        stiffness: 100.0,
+        damping: 20.0,
+        ..SpringConfig::default()
+    };
+    for dt in [1e-12, 0.01, 0.2, 2.0] {
+        let critical = physical(-500., 1234., 100., dt, base);
+        for offset in [-1e-9, -1e-12, 0., 1e-12, 1e-9] {
+            let config = SpringConfig {
+                damping: 20.0 * (1.0 + offset),
+                ..base
+            };
+            let actual = physical(-500., 1234., 100., dt, config);
+            close(actual.0, critical.0, 1e-6);
+            close(actual.1, critical.1, 1e-5);
+        }
+    }
+}
+
+#[test]
+fn normal_trace_records_the_old_numerical_error() {
+    // Captured BEFORE replacing the integrator. Duration rounds to nanoseconds,
+    // giving five old substeps at 60 Hz, not four exact 1/240-second steps.
+    let gold = [
+        (1, 45.440622413787736, 3968.266514512032),
+        (3, 204.05862514520575, 4637.558952650968),
+        (6, 375.42995175442246, 2376.3948049425335),
+        (12, 482.2490947739653, 366.6034676365624),
+        (18, 497.62691927670414, 49.586155106479524),
+    ];
+    let mut old = AnimatedValue::new(0.0);
+    old.set_target(500.0);
+    let mut max_error: f64 = 0.0;
+    for i in 1..=18 {
+        legacy_advance(&mut old, Duration::from_secs_f64(1.0 / 60.0), SpringConfig::default());
+        if let Some((_, x, v)) = gold.iter().find(|(n, _, _)| *n == i) {
+            close(old.current, *x, 1e-9);
+            close(old.velocity, *v, 1e-9);
+        }
+        let dt = Duration::from_secs_f64(1.0 / 60.0).as_secs_f64() * i as f64;
+        let exact = reference(0., 0., 500., dt, SpringConfig::default());
+        max_error = max_error.max((old.current - exact.0).abs());
+    }
+    assert!(
+        max_error > 13.0 && max_error < 14.0,
+        "old integrator error: {max_error}"
+    );
+}
+
+#[test]
+fn retarget_golden_policy_values() {
+    let mut old = AnimatedValue::new(0.0);
+    old.set_target(1000.0);
+    legacy_advance(&mut old, Duration::from_millis(40), SpringConfig::default());
+    close(old.current, 318.94300938241565, 1e-9);
+    close(old.velocity, 9837.123292018367, 1e-9);
+    old.retarget_preserving_motion(2000.);
+    close(old.velocity, 9837.123292018367, 1e-9);
+    old.retarget_preserving_motion(-500.);
+    close(old.velocity, 3442.993152206428, 1e-9);
+}
+
+#[test]
+fn zero_elapsed_time_is_a_strict_noop_even_inside_settling_tolerance() {
+    let mut value = AnimatedValue::new(0.05);
+    value.set_target(0.0);
+    let before = value;
+    value.advance(Duration::ZERO, SpringConfig::default());
+    assert_eq!(value, before);
+}
+
+#[test]
+fn gesture_flick_trace_crosses_once_and_settles_while_clamp_stops_at_crossing() {
+    let config = SpringConfig::default();
+    let mut free = AnimatedValue::new(0.0);
+    free.velocity = 20000.0;
+    free.set_target(100.0);
+    let mut clamped = free;
+    let mut crossings = 0;
+    let mut previous_error: f64 = -100.0;
+    for ms in 1..=1500 {
+        free.advance_with_policy(Duration::from_millis(1), config, CrossingPolicy::AllowOvershoot);
+        clamped.advance_with_policy(Duration::from_millis(1), config, CrossingPolicy::NoCrossing);
+        let error = free.current - free.target;
+        if error != 0.0 && previous_error.signum() != error.signum() {
+            crossings += 1;
+        }
+        if error != 0.0 {
+            previous_error = error;
+        }
+        assert!(clamped.current <= clamped.target);
+        if clamped.current == clamped.target {
+            assert_eq!(clamped.velocity, 0.0);
+        }
+        if ms <= 200 {
+            let exact = reference(0., 20000., 100., ms as f64 / 1000.0, config);
+            close(free.current, exact.0, 1e-7);
+            close(free.velocity, exact.1, 1e-6);
+        }
+    }
+    assert_eq!(crossings, 1);
+    assert_eq!((free.current, free.velocity), (100., 0.));
+    assert_eq!((clamped.current, clamped.velocity), (100., 0.));
+}
+
+#[test]
+fn rapid_retarget_trace_preserves_position_and_the_golden_policy() {
+    let mut clock = FakeClock::new();
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(1000.0);
+    clock.sample(Duration::from_millis(40), &mut value, SpringConfig::default());
+    let before = value;
+    value.retarget_preserving_motion(2000.0);
+    assert_eq!(value.current, before.current);
+    assert_eq!(value.velocity, before.velocity);
+    clock.sample(Duration::from_millis(65), &mut value, SpringConfig::default());
+    let before = value;
+    value.retarget_preserving_motion(-500.0);
+    assert_eq!(value.current, before.current);
+    assert_eq!(value.velocity, before.velocity * 0.35);
+    clock.sample(Duration::from_millis(81), &mut value, SpringConfig::default());
+    assert_ne!(value.velocity, 0.0);
+
+    // Apply the current policy to the exact old in-flight state too.
+    value.current = 318.94300938241565;
+    value.velocity = 9837.123292018367;
+    value.retarget_preserving_motion(-500.0);
+    close(value.velocity, 3442.993152206428, 1e-9);
+}
+
+#[test]
+fn interrupted_resize_and_zoom_targets_are_c0_continuous() {
+    let initial = Rect::new(0., 0., 400., 300.);
+    let mut geometry = WindowGeometry::new(initial, None);
+    geometry.set_logical_target(Rect::new(100., 50., 900., 600.), Duration::ZERO);
+    geometry.advance(Duration::from_millis(40), SpringConfig::default(), true);
+    for mode in [
+        PresentationMode::Normal,
+        PresentationMode::Maximized,
+        PresentationMode::Fullscreen,
+        PresentationMode::Normal,
+    ] {
+        let before = (geometry.visual.current, geometry.decorations);
+        geometry.set_presentation_mode(Rect::new(200., 100., 700., 500.), mode, Duration::from_millis(40));
+        assert_eq!((geometry.visual.current, geometry.decorations), before);
+        geometry.advance(Duration::ZERO, SpringConfig::default(), true);
+        assert_eq!((geometry.visual.current, geometry.decorations), before);
+        geometry.advance(Duration::from_millis(16), SpringConfig::default(), true);
+    }
+}
+
+#[test]
+fn variable_cadence_traces_match_reference_at_every_absolute_timestamp() {
+    let config = SpringConfig::default();
+    let mut schedules = vec![vec![16600], vec![8300], vec![5000, 22000, 11000, 37000]];
+    for mut seed in [1u64, 0x12345678, 0xdeadbeef] {
+        let mut seeded = Vec::new();
+        for _ in 0..64 {
+            seed = seed.wrapping_mul(6364136223846793005).wrapping_add(1);
+            seeded.push((1 + (seed >> 32) % 47) * 1000);
+        }
+        schedules.push(seeded);
+    }
+
+    for schedule in schedules {
+        let mut clock = FakeClock::new();
+        let mut value = AnimatedValue::new(-100.0);
+        value.set_target(500.0);
+        let mut elapsed = Duration::ZERO;
+        let mut settled_at = None;
+        for i in 0..500 {
+            elapsed += Duration::from_micros(schedule[i % schedule.len()]);
+            let sample = clock.sample(elapsed, &mut value, config);
+            let exact = reference(-100., 0., 500., elapsed.as_secs_f64(), config);
+            if sample.current == sample.target && sample.velocity == 0.0 {
+                assert_eq!((sample.current, sample.velocity), (500., 0.));
+                settled_at.get_or_insert(elapsed);
+            } else {
+                close(sample.current, exact.0, config.position_tolerance);
+                close(sample.velocity, exact.1, config.velocity_tolerance);
+            }
+            if elapsed >= Duration::from_secs(2) {
+                break;
+            }
+        }
+        let settled_at = settled_at.unwrap();
+        let mut reference_settle_us = 0;
+        while {
+            let (x, v) = reference(-100., 0., 500., reference_settle_us as f64 / 1e6, config);
+            (x - 500.).abs() > config.position_tolerance || v.abs() > config.velocity_tolerance
+        } {
+            reference_settle_us += 100;
+        }
+        let exact_settle = Duration::from_micros(reference_settle_us);
+        assert!(settled_at.abs_diff(exact_settle) <= Duration::from_micros(*schedule.iter().max().unwrap() + 100));
+    }
+}
+
+#[test]
+fn scalar_and_rect_share_math_and_rect_clamps_only_the_crossing_component() {
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(100.0);
+    value.velocity = 20000.0;
+    let mut rect = AnimatedRect::new(Rect::new(0., 0., 400., 300.));
+    rect.set_target(Rect::new(100., 500., 400., 300.));
+    rect.velocity.x = value.velocity;
+    for policy in [CrossingPolicy::NoCrossing, CrossingPolicy::AllowOvershoot] {
+        let mut scalar = value;
+        let mut shape = rect;
+        scalar.advance_with_policy(Duration::from_millis(10), SpringConfig::default(), policy);
+        shape.advance_with_policy(Duration::from_millis(10), SpringConfig::default(), policy);
+        assert_eq!(scalar.current, shape.current.x);
+        assert_eq!(scalar.velocity, shape.velocity.x);
+        assert!(shape.current.y > 0.0 && shape.current.y < 500.0);
+        if policy == CrossingPolicy::NoCrossing {
+            assert_eq!(shape.current.x, 100.0);
+            assert_eq!(shape.velocity.x, 0.0);
+            assert_ne!(shape.velocity.y, 0.0);
+        }
+    }
+}
+
+#[test]
+fn clamp_detects_crossing_inside_a_stall_even_when_endpoints_have_same_sign() {
+    let config = SpringConfig {
+        stiffness: 100.0,
+        damping: 0.0,
+        ..SpringConfig::default()
+    };
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(100.0);
+    value.advance_with_policy(
+        Duration::from_secs_f64(std::f64::consts::TAU / 10.0),
+        config,
+        CrossingPolicy::NoCrossing,
+    );
+    assert_eq!((value.current, value.velocity), (100., 0.));
+}
+
+#[test]
+fn solver_is_finite_for_realistic_ranges_and_invalid_configuration_falls_back() {
+    for mass in [0.01, 1.0, 100.0] {
+        for stiffness in [0.1, 700.0, 100000.0] {
+            for damping in [0.0, 0.01, 53.0, 1000000.0] {
+                for dt in [1e-12, 0.008, 0.4, 10.0, 3600.0] {
+                    let config = SpringConfig {
+                        mass,
+                        stiffness,
+                        damping,
+                        ..SpringConfig::default()
+                    };
+                    let (x, v) = physical(-10000., 50000., 10000., dt, config);
+                    assert!(x.is_finite() && v.is_finite(), "{config:?} dt {dt}: {x} {v}");
+                }
+            }
+        }
+    }
+    for bad in [0.0, -1.0, f64::NAN, f64::INFINITY] {
+        for config in [
+            SpringConfig {
+                mass: bad,
+                ..SpringConfig::default()
+            },
+            SpringConfig {
+                stiffness: bad,
+                ..SpringConfig::default()
+            },
+        ] {
+            assert_eq!(
+                physical(0., 10., 100., 0.1, config),
+                physical(0., 10., 100., 0.1, SpringConfig::default())
+            );
+        }
+    }
+
+    for bad in [-1.0, f64::NAN, f64::INFINITY] {
+        let config = SpringConfig {
+            damping: bad,
+            ..SpringConfig::default()
+        };
+        assert_eq!(
+            physical(0., 10., 100., 0.1, config),
+            physical(0., 10., 100., 0.1, SpringConfig::default())
+        );
+    }
+}
+
+#[test]
+fn physical_zero_and_negative_time_do_not_move_state() {
+    for dt in [0.0, -0.01] {
+        assert_eq!(physical(5.0, 100.0, 10.0, dt, SpringConfig::default()), (5.0, 100.0));
+    }
+}
+
+#[test]
+fn resize_target_rebases_an_existing_zoom_without_a_zero_time_jump() {
+    let mut geometry = WindowGeometry::new(Rect::new(100., 50., 400., 300.), None);
+    geometry.set_presentation_mode(
+        Rect::new(0., 0., 1920., 1080.),
+        PresentationMode::Fullscreen,
+        Duration::ZERO,
+    );
+    geometry.advance(Duration::from_millis(40), SpringConfig::default(), true);
+    let before = (geometry.visual.current, geometry.decorations);
+    geometry.set_logical_target(Rect::new(0., 0., 1600., 900.), Duration::from_millis(40));
+    geometry.advance(Duration::ZERO, SpringConfig::default(), true);
+    assert_eq!((geometry.visual.current, geometry.decorations), before);
+    geometry.advance(Duration::from_nanos(1), SpringConfig::default(), true);
+    close(geometry.visual.current.width, before.0.width, 1e-4);
+}
+
+#[test]
+fn small_default_normal_motion_matches_legacy_within_half_a_pixel() {
+    let mut old = AnimatedValue::new(0.0);
+    old.set_target(10.0);
+    let mut clock = FakeClock::new();
+    let mut exact = old;
+    for i in 1..=120 {
+        let step = Duration::from_secs_f64(1.0 / 60.0);
+        legacy_advance(&mut old, step, SpringConfig::default());
+        let sample = clock.sample(step * i, &mut exact, SpringConfig::default());
+        close(sample.current, old.current, 0.5);
+    }
+}
+
+#[test]
+fn creation_clock_excludes_earlier_idle_time_and_records_all_sample_fields() {
+    let mut clock = FakeClock::new();
+    let created = Duration::from_secs(10);
+    clock.resume(created);
+    let mut value = AnimatedValue::new(0.0);
+    value.set_target(500.0);
+    let first = clock.sample(created + Duration::from_millis(16), &mut value, SpringConfig::default());
+    let exact = reference(0.0, 0.0, 500.0, 0.016, SpringConfig::default());
+    close(first.current, exact.0, 1e-8);
+    close(first.velocity, exact.1, 1e-8);
+    assert_eq!(first.time, created + Duration::from_millis(16));
+    assert_eq!(first.target, 500.0);
+    assert_eq!(clock.trace.len(), 1);
+    value.retarget_preserving_motion(800.0);
+    let same_time = clock.sample(first.time, &mut value, SpringConfig::default());
+    assert_eq!(same_time.current, first.current);
+    assert_eq!(same_time.velocity, first.velocity);
+    assert_eq!(same_time.target, 800.0);
+}
+
+#[test]
+fn damped_physical_solutions_converge_in_all_three_regimes() {
+    for damping in [5.0, 20.0, 53.0, 2000.0] {
+        let config = SpringConfig {
+            stiffness: 100.0,
+            damping,
+            ..SpringConfig::default()
+        };
+        let (x, v) = physical(-500.0, 1234.0, 100.0, 2000.0, config);
+        close(x, 100.0, 1e-9);
+        close(v, 0.0, 1e-9);
+    }
+}
+
+#[test]
+fn solver_is_continuous_on_both_sides_of_the_flick_crossing() {
+    let config = SpringConfig::default();
+    let (mut low, mut high) = (0.0, 0.02);
+    for _ in 0..60 {
+        let middle = (low + high) / 2.0;
+        if physical(0.0, 20000.0, 100.0, middle, config).0 < 100.0 {
+            low = middle;
+        } else {
+            high = middle;
+        }
+    }
+    let before = physical(0.0, 20000.0, 100.0, low - 1e-9, config);
+    let after = physical(0.0, 20000.0, 100.0, high + 1e-9, config);
+    assert!(before.0 < 100.0 && after.0 > 100.0);
+    close(before.0, after.0, 1e-4);
+    close(before.1, after.1, 0.01);
+    assert!(after.1 > 1000.0);
+}

docs/installation.md

@@ -244,6 +244,7 @@ Installer options:
 ```sh
 ./scripts/install.sh --dry-run
 ./scripts/install.sh --offline --release-id my-build
+./scripts/install.sh --resize-metrics
 ./scripts/install.sh --skip-build
 ```
 
@@ -252,6 +253,10 @@ cached, or `--skip-build` after building every component from the revision you w
 install. The latter uses binaries already in `target/release/`, and
 `./scripts/install.sh --help` lists all available options.
 
+Use `--resize-metrics` to enable resize-barrier counters and duration histograms in
+the release build. Ferese prints the measurements to its session log on normal exit.
+This option requires building and cannot be combined with `--skip-build`.
+
 ## Logout and recovery
 
 Save your work before leaving: **Super+Shift+E** and `feresectl request-logout` ask for

scripts/install.sh

@@ -13,6 +13,7 @@ Run from any directory as your normal user. Existing user config is preserved.
   --skip-build     Install existing target/release binaries
   --replace-portal-config  Back up and replace existing Ferese portal configuration
   --offline        Build using only cached Cargo dependencies
+  --resize-metrics Build with resize-barrier measurements (reported on exit)
   --dry-run        Print commands without building or installing
   -h, --help       Show this help
 
@@ -48,6 +49,7 @@ release_id="$(date -u +%Y%m%d-%H%M%S)-$$"
 skip_build=false
 replace_portal_config=false
 offline=false
+resize_metrics=false
 dry_run=false
 while (($#)); do
     case $1 in
@@ -59,12 +61,17 @@ while (($#)); do
         --skip-build) skip_build=true; shift ;;
         --replace-portal-config) replace_portal_config=true; shift ;;
         --offline) offline=true; shift ;;
+        --resize-metrics) resize_metrics=true; shift ;;
         --dry-run) dry_run=true; shift ;;
         -h|--help) usage; exit 0 ;;
         *) fail "unknown option: $1 (see --help)" ;;
     esac
 done
 [[ $release_id =~ ^[a-zA-Z0-9][a-zA-Z0-9._-]*$ ]] || fail 'invalid release ID'
+if $skip_build && $resize_metrics; then
+    fail '--resize-metrics requires a build; omit --skip-build'
+fi
+
 [[ $(uname -s) == Linux ]] || fail 'Ferese requires Linux'
 for tool in desktop-file-validate dbus-run-session; do
     command -v "$tool" >/dev/null || fail "required command not found: $tool"
@@ -79,6 +86,7 @@ build=(cargo build --manifest-path "$repo_dir/Cargo.toml"
     --target-dir "$repo_dir/target" --release --locked
     -p ferese -p ferese-shell -p ferese-settings -p feresectl -p ferese-lock -p ferese-polkit-agent -p xdg-desktop-portal-ferese)
 if $offline; then build+=(--offline); fi
+if $resize_metrics; then build+=(--features ferese/resize-metrics); fi
 if ! $skip_build; then
     ((EUID != 0)) || fail 'build as your normal user, without sudo (or use --skip-build)'
     command -v cargo >/dev/null || fail 'install the Rust toolchain and Cargo first'

scripts/tests/test_portal_isolated.py

@@ -101,6 +101,23 @@ class PortalTest(unittest.TestCase):
                 connection = Gio.DBusConnection.new_for_address_sync(env["DBUS_SESSION_BUS_ADDRESS"],
                     Gio.DBusConnectionFlags.AUTHENTICATION_CLIENT | Gio.DBusConnectionFlags.MESSAGE_BUS_CONNECTION, None, None)
                 backend = launch("backend", [str(binary)])
+                # The frontend can D-Bus-activate an installed backend if it
+                # starts before this test's binary owns its service name.
+                def backend_ready():
+                    bus_name = "org.freedesktop.DBus"
+                    impl_name = "org.freedesktop.impl.portal.desktop.ferese"
+                    owner = connection.call_sync(bus_name, "/org/freedesktop/DBus", bus_name,
+                        "NameHasOwner", GLib.Variant("(s)", (impl_name,)), GLib.VariantType("(b)"),
+                        Gio.DBusCallFlags.NONE, 1000, None).unpack()[0]
+                    if not owner:
+                        return False
+
+                    pid = connection.call_sync(bus_name, "/org/freedesktop/DBus", bus_name,
+                        "GetConnectionUnixProcessID", GLib.Variant("(s)", (impl_name,)), GLib.VariantType("(u)"),
+                        Gio.DBusCallFlags.NONE, 1000, None).unpack()[0]
+                    return pid == backend.pid
+
+                until(backend_ready)
                 frontend = launch("frontend", [os.environ.get("FERESE_TEST_PORTAL_FRONTEND", "/usr/libexec/xdg-desktop-portal")])
                 name, path, iface = "org.freedesktop.portal.Desktop", "/org/freedesktop/portal/desktop", "org.freedesktop.portal.ScreenCast"
                 responses = {}