[omartelo]

Partial redraw trusted EGL_BUFFER_AGE, and NVIDIA 595 on COSMIC reports an age its buffer does not hold, so the whole window showed stale pixels. This keeps the frame in an sRGB texture the renderer owns, redraws only the damaged rectangles there, and blits it whole to the output with a plain full swap. Buffer age and swap-with-damage are gone. The behaviour stays behind GIO_DAMAGE=on.

Non-OpenGL backends keep drawing whole frames straight to the output (FeatureBlitFramebuffer), as does any frame without a clear or with a translucent clear, whose ground would blend over the kept frame. Paths the rectangles do not reach keep their place in the atlas and only lose their stencil, because a path's place changes its coverage by a rounding.

A second commit logs once when GL_RENDERER names a software renderer.

Pixel test

gpu/headless/retained_test.go draws 12 seeds of 30 frames changing a little at a time into the retained framebuffer and compares each frame with the same frame drawn whole straight to the output, dropping the retained contents every 11 frames (resize) and requiring a whole redraw. Without stencilled paths the frames match byte for byte. With paths a channel may differ by 1: whole frames drawn with their paths at other atlas positions differ from each other by the same one level (measured, 185 of 3,072,000 bytes across 10 seeds, max delta 1), so byte equality against a whole frame is not defined there.

On a real NVIDIA surface, a private headless compositor screenshot of the same window with GIO_DAMAGE off and on has 0 differing pixels.

Measurements

Real NVIDIA GPU, private headless umbriel, one shell session, 1260x700. GL work per frame is GPU and driver time for the passes and the blit, between two glFinish calls, from an uncommitted local instrumentation; render is LICH_NATIVE_FRAME_LOG.

Load	Mode	GL work (ms, 240-frame buckets)	render p50 / p95 (ms)
One line rewritten, 19 fps	whole	2.04-3.60	3.6 / 7.0
One line rewritten, 19 fps	retained	1.24-2.05	2.6 / 5.2
Scrolling log, 10 fps, 66% of the window dirty	whole	4.50-4.69	6.0 / 11
Scrolling log, 10 fps, 66% of the window dirty	retained	4.74-4.88	6.4 / 11.1

The retained texture costs width x height x 4 bytes (3.5 MB at 1260x700). The tracker adds about 47 KiB of allocations per frame.

Test plan

• go test ./gpu/... ./app/... ./internal/..., also TestPartialFramesMatchWhole and TestPartialPathFramesMatchWhole on NVIDIA and on llvmpipe, 5 repeats on NVIDIA
• Mutations: removing the ground and removing the blit both fail the pixel test
• GOOS=windows go build ./..., GOOS=js GOARCH=wasm go build ./gpu/... ./internal/gl/...
• lich native/ go vet ./... and go test ./... with replace gioui.org pointed at this branch