From e57d355db226efca4235e57b04dde0841b292686 Mon Sep 17 00:00:00 2001
From: Samuel Onoja <samiodev@icloud.com>
Date: Fri, 25 Sep 2026 00:44:43 +0100
Subject: [PATCH] feat: support xdg foreign parenting

---
 compositor/src/handlers/mod.rs       |  8 ++++++
 compositor/src/handlers/xdg_shell.rs | 25 ++++++++++++++++++
 compositor/src/state.rs              | 39 ++++++++++++++++++++++++++++
 3 files changed, 72 insertions(+)

diff --git a/compositor/src/handlers/mod.rs b/compositor/src/handlers/mod.rs
index c16bd8b..c92c366 100644
--- a/compositor/src/handlers/mod.rs
+++ b/compositor/src/handlers/mod.rs
@@ -27,6 +27,7 @@ use smithay::{
                 PrimarySelectionHandler, PrimarySelectionState, set_primary_focus,
             },
         },
+        xdg_foreign::{XdgForeignHandler, XdgForeignState},
     },
 };
 
@@ -150,6 +151,12 @@ impl PrimarySelectionHandler for Ferese {
     }
 }
 
+impl XdgForeignHandler for Ferese {
+    fn xdg_foreign_state(&mut self) -> &mut XdgForeignState {
+        &mut self.xdg_foreign_state
+    }
+}
+
 smithay::delegate_compositor!(Ferese);
 smithay::delegate_data_device!(Ferese);
 smithay::delegate_fractional_scale!(Ferese);
@@ -165,4 +172,5 @@ smithay::delegate_single_pixel_buffer!(Ferese);
 smithay::delegate_viewporter!(Ferese);
 smithay::delegate_xdg_activation!(Ferese);
 smithay::delegate_xdg_decoration!(Ferese);
+smithay::delegate_xdg_foreign!(Ferese);
 smithay::delegate_xdg_shell!(Ferese);
diff --git a/compositor/src/handlers/xdg_shell.rs b/compositor/src/handlers/xdg_shell.rs
index 384cf4d..10b0e5e 100644
--- a/compositor/src/handlers/xdg_shell.rs
+++ b/compositor/src/handlers/xdg_shell.rs
@@ -51,6 +51,31 @@ impl XdgShellHandler for Ferese {
         }
     }
 
+    fn parent_changed(&mut self, surface: ToplevelSurface) {
+        let Some(parent) = surface.parent() else {
+            return;
+        };
+        let parent = self.window_ids.iter().find_map(|(window, id)| {
+            window
+                .toplevel()
+                .is_some_and(|toplevel| toplevel.wl_surface() == &parent)
+                .then_some(*id)
+        });
+        let child = self
+            .space
+            .elements()
+            .find(|window| {
+                window
+                    .toplevel()
+                    .is_some_and(|toplevel| toplevel.wl_surface() == surface.wl_surface())
+            })
+            .cloned();
+
+        if let (Some(parent), Some(child)) = (parent, child) {
+            self.make_window_transient(&child, parent);
+        }
+    }
+
     fn toplevel_destroyed(&mut self, surface: ToplevelSurface) {
         let Some(window) = self
             .space
diff --git a/compositor/src/state.rs b/compositor/src/state.rs
index b077d4c..46ed2bc 100644
--- a/compositor/src/state.rs
+++ b/compositor/src/state.rs
@@ -45,6 +45,7 @@ use smithay::{
         socket::ListeningSocketSource,
         viewporter::ViewporterState,
         xdg_activation::XdgActivationState,
+        xdg_foreign::XdgForeignState,
     },
 };
 
@@ -80,6 +81,7 @@ pub struct Ferese {
     pub viewporter_state: ViewporterState,
     pub layer_shell_state: WlrLayerShellState,
     pub xdg_activation_state: XdgActivationState,
+    pub xdg_foreign_state: XdgForeignState,
     pub xdg_shell_state: XdgShellState,
 }
 
@@ -105,6 +107,7 @@ impl Ferese {
         let viewporter_state = ViewporterState::new::<Self>(&display_handle);
         let layer_shell_state = WlrLayerShellState::new::<Self>(&display_handle);
         let xdg_activation_state = XdgActivationState::new::<Self>(&display_handle);
+        let xdg_foreign_state = XdgForeignState::new::<Self>(&display_handle);
         let mut seat_state = SeatState::new();
         let mut seat = seat_state.new_wl_seat(&display_handle, "ferese-winit");
         seat.add_keyboard(Default::default(), 200, 25)?;
@@ -145,6 +148,7 @@ impl Ferese {
             viewporter_state,
             layer_shell_state,
             xdg_activation_state,
+            xdg_foreign_state,
             xdg_shell_state,
         })
     }
@@ -347,6 +351,41 @@ impl Ferese {
         self.relayout();
     }
 
+    pub fn make_window_transient(&mut self, window: &Window, parent: WindowId) {
+        let Some(id) = self.window_ids.get(window).copied() else {
+            return;
+        };
+        if self.workspaces.workspace_for_window(id) != self.workspaces.workspace_for_window(parent)
+        {
+            tracing::warn!(
+                ?id,
+                ?parent,
+                "ignored transient parent on another workspace"
+            );
+            return;
+        }
+        let Some(bounds) = self.output_bounds() else {
+            return;
+        };
+        let parent_rect = self.logical_window_rect(parent, bounds).unwrap_or(bounds);
+        let rect = centered_transient_rect(parent_rect);
+        let result = match self.workspaces.placement(id) {
+            Some(WindowPlacement::Tiled) => self
+                .workspaces
+                .toggle_floating(id, rect, Axis::Horizontal, 0.5)
+                .map(|_| ()),
+            Some(WindowPlacement::Floating { .. }) => self.workspaces.set_floating_rect(id, rect),
+            None => return,
+        };
+
+        if let Err(error) = result {
+            tracing::warn!(%error, ?id, ?parent, "failed to apply transient placement");
+            return;
+        }
+
+        self.relayout();
+    }
+
     pub fn remove_tiled_window(&mut self, window: &Window) {
         let Some(id) = self.window_ids.remove(window) else {
             return;