feresaria/docs/nested-resize-testing.md

Nested live-resize regression check

The shell enables libcosmic's wgpu feature. Without it, iced uses tiny-skia to resample and rasterize the wallpaper on the CPU. In an unoptimized build, this blocks the UI/render thread shared with the top bar for seconds. Prompt layer configures, buffer stretching, and compositor vsync cannot fix that client-side bottleneck. Software rendering remains available as a fallback.

Reproduce and compare

cargo build --locked -p ferese -p ferese-shell
WAYLAND_DEBUG=1 FERESE_TRACE_PERFORMANCE=1 \
  target/debug/ferese --backend nested --grant-effects --grant-shell-control -- \
  target/debug/ferese-shell 2>/tmp/ferese-resize.log

Use a configured wallpaper. Drag the host window's edges in both directions, then maximize/restore it repeatedly. Check that the wallpaper and bar follow the window, including at fractional host scaling. Repeat with ICED_BACKEND=wgpu to require GPU rendering (rather than silently accepting fallback), and with ICED_BACKEND=tiny-skia to compare software rendering. Use a release build if software fallback is needed for regular use.

Trace the chain, not just the compositor's frame rate:

1. Host xdg_toplevel.configure becomes Winit Resized.
2. Ferese updates the output, arranges layers, and sends zwlr_layer_surface_v1.configure for the new logical dimensions.
3. The shell acknowledges configure and eventually attaches/commits a new buffer. A fast acknowledgement does not imply a finished render.
4. Committed buffers change surface damage; Ferese renders the damaged output and submits through EGL to the host compositor.

FERESE_TRACE_PERFORMANCE measures Ferese's rendering/submission, not shell rasterization. Wayland traces distinguish software SHM buffers from GPU buffers (Mesa Vulkan on the tested system).

Measured validation (KDE Wayland, 175% scale)

With temporary instrumentation, 24 host maximize/restore requests spaced 250 ms apart generated 26 Winit resize events including initialization. The same debug binaries were tested with each renderer forced:

Measurement	tiny-skia	wgpu
Median configure acknowledgement	1.35 ms	2.27 ms
Median Ferese render + submit	0.82 ms	1.10 ms
Wallpaper buffer interval	5919 ms (only two buffers)	9.85 ms median
Bar buffers observed	1	996

The CPU capture ran for 14 seconds and the GPU capture for 18 seconds; counts are diagnostic, not equal-duration throughput benchmarks. GPU maximum buffer intervals were about 200 ms for wallpaper and 158 ms for the bar, so the median does not imply every frame met a refresh deadline. These figures include protocol tracing overhead. The temporary resize/timing hooks were removed after testing; no synthetic resizing runs in production.

The fix does not alter output coordinates, layer exclusion, damage tracking, frame scheduling, or EGL swap settings. It also applies to the same shell running under DRM, though this validation used the nested backend.