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
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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