test_drawWindow_widget_layers.html test failed with WebRender.

https://treeherder.mozilla.org/#/jobs?repo=try&selectedJob=225147839&revision=49dff9bfd0746b6eb377235634f6a843956959d9

When I run the test locally with the following command, it always succeeded. But it failed on try tests.

MOZ_ACCELERATED=1 MOZ_WEBRENDER=1 ./mach test dom/canvas/test/chrome/test_drawWindow_widget_layers.html

From attachment 9040327 [details], it seems that the readback image lost transparency.

The image was readback via CanvasRenderingContext2D::DrawWindow()
https://searchfox.org/mozilla-central/rev/b6f330e5234582eb921559dbfc852d263e14d93b/content/canvas/src/CanvasRenderingContext2D.cpp#3170

The DrawWindow() was called from
https://searchfox.org/mozilla-central/source/dom/canvas/test/file_drawWindow_common.js

When I locally tested, GLContextGLX::FindVisual() was called with requesting alpha in nsWindow::Create().
https://searchfox.org/mozilla-central/rev/956e93c403573cead1717b774e924cfee561efa9/widget/gtk/nsWindow.cpp#3256

But on try test needsAlphaVisual was false. When it was false, GLContextGLX::FindVisual() chooses RGB(24bit) visual.
https://searchfox.org/mozilla-central/rev/187dfafc29f4cc692b724b544def376c4de761fd/gfx/gl/GLContextProviderGLX.cpp#1172

(In reply to Sotaro Ikeda [:sotaro] from comment #5)

But on try test needsAlphaVisual was false. When it was false, GLContextGLX::FindVisual() chooses RGB(24bit) visual.
https://searchfox.org/mozilla-central/rev/187dfafc29f4cc692b724b544def376c4de761fd/gfx/gl/GLContextProviderGLX.cpp#1172

When I set needsAlphaVisual = false, the test failed locally and captured image was like attachment 9040327 [details]. From it, when visual was RGB(24bit), readback image looses alpha channel.

attachment 9040331 [details] [diff] [review]　addressed the test failure.
https://treeherder.mozilla.org/#/jobs?repo=try&revision=44c99a979023675c4679f24159b29b75d7935db9

When hardware acceleration is enabled, GLContextGLX::FindVisual() is used to choose visual. When widget does not request AlphaVisual, the FindVisual() always choose RGB(24bit) visual. It causes to loose alpha during readback.

https://hg.mozilla.org/mozilla-central/rev/2ce3a517eb67