fix: unify animated presentation and stacking

docs/animation-validation.md

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+# Animation regression checks
+
+Run `cargo test -p ferese-animation -p ferese --offline` for transition,
+resize pacing, reduced motion, stacking, and refresh-rate regression tests.
+
+For visual validation, use a disposable Ferese session with three tiled windows
+containing text and at least one floating window. Test both the nested and DRM
+backends. Record the session at its native refresh rate when comparing builds;
+single screenshots cannot establish smoothness.
+
+| Scenario | Expected result |
+| --- | --- |
+| Super+F on the middle column, then exit | Neighbours remain behind the expanding window. The shrinking window stays above them until the transition finishes. |
+| Toggle Super+F repeatedly at 40–80 ms intervals | Every reversal starts at the presented frame. Corners, border and shadow follow the same zoom progress. The final state matches the last input. |
+| Cycle width, then immediately Super+F | The old column-width spring cannot override fullscreen width. |
+| Exit fullscreen and immediately resize or scroll | The current frame is retained when the target changes; scrolling resumes without a position jump. |
+| Drag a floating window; resize from its top-left corner | The window follows the pointer directly. The opposite edge stays anchored. Delayed client commits do not move or raise it. |
+| Zoom a slow-redrawing client | Intermediate resize requests wait for commits or timeout. The final size is always requested and the animation keeps moving. |
+| Disable borders and shadows, then zoom | Rounded corner pixels repaint through the final frame; no corner remnants remain. |
+| Repeat on 60 Hz and 144 Hz outputs; move focus between them | Similar elapsed-time motion with no double advancement. Check frame timing on actual DRM hardware. |
+| Enable reduced motion | Geometry and decorations reach their final values together. |
+
+The zoom resize policy asks responsive clients to redraw near the current visual
+size. A late client buffer is still stretched as a fallback; this is not a buffer
+snapshot or a guarantee of perfectly sharp text on every intermediate frame.