The layers system gets confused when we have a nonscrollable solid color behind the scrollable canvas background. I need to fuse the nonscrollable viewport fallback background color with the canvas background when possible. The attached patch does this by giving nsCanvasBackgroundDisplayItem the ability to paint an extra solid color, and by making AddCanvasBackgroundColorItem locate a suitable nsCanvasBackgroundDisplayItem and poke the solid color directly into that item if possible. This means we don't need to build the SolidColor display item at all in most cases.

Comment on attachment 451832 [details] [diff] [review] fix Should probably add a comment to all the other AddCanvasBackgroundColorItem call sites that it is called after the main display list is constructed for a reason like you have in nsFrameFrame.cpp (and check that it is indeed called after in all places). Can you update the comment in nsCSSRendering::PaintBackgroundWithSC above "PRBool isCanvasFrame = ..." saying that the background color is drawn by the canvas background item or solid color item? AddCanvasBackgroundColor returns a bool, I think you should check that in AddCanvasBackgroundColorItem, and use the regular path if it returns false. + virtual PRBool IsUniform(nsDisplayListBuilder* aBuilder, nscolor* aColor) + { + nscolor background; + if (!nsDisplayBackground::IsUniform(aBuilder, &background)) + return PR_FALSE; + *aColor = NS_ComposeColors(mExtraBackgroundColor, background); + return PR_TRUE; + } If everything is working right then I think mExtraBackgroundColor should equal background, and we should be drawing just one, not the composition of both. r=tnikkel if you agree with these comments.

(In reply to comment #1) > If everything is working right then I think mExtraBackgroundColor should equal > background, and we should be drawing just one, not the composition of both. Oops, I didn't see how nsDisplayBackground::IsUniform works. Can you just assert that background is completely transparent and not compose?

(In reply to comment #1) > Should probably add a comment to all the other AddCanvasBackgroundColorItem > call sites that it is called after the main display list is constructed for a > reason like you have in nsFrameFrame.cpp (and check that it is indeed called > after in all places). Done. There's only one other, in nsLayoutUtils::PaintFrame. > Can you update the comment in nsCSSRendering::PaintBackgroundWithSC above > "PRBool isCanvasFrame = ..." saying that the background color is drawn by the > canvas background item or solid color item? Done. > AddCanvasBackgroundColor returns a bool, I think you should check that in > AddCanvasBackgroundColorItem, and use the regular path if it returns false. Yes indeed. Done. > Oops, I didn't see how nsDisplayBackground::IsUniform works. Can you just > assert that background is completely transparent and not compose? Yes. Done. Still sr+ with that?

Sounds good, the r=tnikkel is still valid!