[yiguodev]

Summary

Fix a GTK layout/repaint loop when hiding a Linux window's title bar. GTK rewrites CSD shadow widths during allocation; declaring a second set of widths from the allocation callback keeps scheduling more layout work.

CSD:     GTK decoration margin owns the shadow gutter
non-CSD: declare GDK extents only when the surface or extents change

Keep CSD content sizing and frameless behavior consistent with GTK, invalidate fallback declarations across surface/style/scale changes, and add a real GTK regression test.

Evidence

• Before: a static Flutter window reproduced the loop without application services. Steady-state main-thread CPU was approximately 96–100%; the new regression fails against unmodified upstream.
• After: the GTK regression passes on Wayland and X11, including unchanged/changed extents, scale notifications, surface recreation, resize, remap, and decoration restoration.
• Rebuilt a downstream Flutter application against this core checkout on GNOME Wayland: all 28 checks pass for startup, rasterized frames, in-app navigation, resize, maximize/restore, hide/show, and return to idle. No preload interceptor or X11 fallback was used.
• git diff --check passes.

Run the native regression with:

cmake -S . -B build -DNATIVEAPI_BUILD_EXAMPLES=OFF -DBUILD_TESTING=ON
cmake --build build --target window_shadow_gutter_linux_test
GDK_BACKEND=wayland ctest --test-dir build -R '^window_shadow_gutter_linux_test$' --output-on-failure
GDK_BACKEND=x11 ctest --test-dir build -R '^window_shadow_gutter_linux_test$' --output-on-failure

Merge Danger

Door: Two-way.

Blast Radius: Linux GTK window geometry.

No public API/ABI or persisted-data changes. The live application checks were on GNOME Wayland; KWin/Hyprland and physical mixed-DPI transitions were not tested.

[chatgpt-codex-connector]

Codex Review Summary: ✅ Completed

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-08T07:06:25.162630Z	1a4c41b	New commits

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Reviewed commit: 75306a011e

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