So when I turn on the flag nothing looks broken to me. Should we just turn it on on Linux and Windows and see what happens, similar to what ccameron did for Mac ( Issue 845213 )?

(I tested only Linux so far. Building on Windows right now.)

I expect it will be more work than that. There are many surfaces in the UI. And I think it's worth doing a finch trial and make sure that there are adequate tests for both visual correctness and performance.

I created a CL to turn on this flag and sent it to trybots to see how angry they are. Only one test is failing, LayerWithRealCompositorTest.DrawAlphaBlendedPixels. Seems like every pixel is slightly off. I'm not sure if we should worry about this? I'll investigate.

Pixel error at x=87 y=168; actual RGBA=(217,3,31,255); expected RGBA=(216,3,32,255)

It's probably just some rounding issue, i.e. rounding up vs down.

Agreed that we should do a finch trial here, but I am also not terribly worried here.  We've been running gpu raster on most platforms already, so the correctness concerns seem unlikely.

It is definitely expected that blending will work differently between software and gpu raster, and I would suggest adding some fuzziness to these tests and allow for rgb values that are +/- 1.

GPU raster for UI is expected to be functional.
We should take a hard look at the perf though. The UI is not build with GPU raster in mind, and the way it does many things will be by uploading bitmaps. We should make sure that we don't regress things. Not sure that we have good perf tests in the area.

***UI Mass Triage***

Adding appropriate label for expert review.