NEW 202692
transform/transform-origin not taken into account in RenderLayer::intersectsDamageRect() 
https://bugs.webkit.org/show_bug.cgi?id=202692
Summary transform/transform-origin not taken into account in RenderLayer::intersectsD...
Nikolas Zimmermann
Reported 2019-10-08 05:36:38 PDT
Testcase: <!DOCTYPE html> <html> <body> <div style="position: absolute; transform-origin: 0 0; transform: rotate(90deg); left: 100px; top: -100px; width: 150px; height: 50px; background-color: green"> </div> </body> </html> Content-Type: text/plain layer at (0,0) size 800x600 RenderView at (0,0) size 800x600 layer at (0,0) size 800x8 RenderBlock {HTML} at (0,0) size 800x8 [color=#3D3D3D] RenderBody {BODY} at (8,8) size 784x0 #EOF #EOF #EOF The object is correctly painted, but missing from the dump. This happens because the writeLayers() function in RenderTreeAsText.cpp incorrectly flags the layer with shouldPaint=false: bool shouldPaint = (behavior.contains(RenderAsTextFlag::ShowAllLayers)) ? true : layer.intersectsDamageRect(layerBounds, damageRect.rect(), &rootLayer, layer.offsetFromAncestor(&rootLayer)); I've added some logging to clarify why: LOG_WITH_STREAM(SVG, stream << " shouldPaint=" << shouldPaint << " layerBounds=" << layerBounds << " damageRect=" << damageRect.rect() << " offfsetFromAnc=" << layer.offsetFromAncestor(&rootLayer);); --> shouldPaint=0 layerBounds=(100,-100) width=150 height=50 damageRect=(0,0) width=800 height=600 offfsetFromAnc=width=100.00 height=-100.00 So indeed, RenderLayer::intersectsDamageRect() incorrectly returns false in this case. Why? RenderLayer::intersectsDamageRect() calls "boundingBox(rootLayer, offsetFromRoot).intersects(damageRect)". RenderLayer::boundingBox() calls RenderLayer::localBoundingBox() (which simply returns the visualOverflowRect() -> (0,0,150,50)) and moves the rect by the passed in "offsetFromRoot" (100,-100): For the testcase above this results in a a LayoutRect(100, -100, 150, 50), which clearly does not intersect with LayoutRect(0, 0, 800, 600) (damageRect). The transform-origin is completely ignored here but plays an important role. With the default transform-origin (50% 50%) the <div> above is invisible, and thus correctly flagged as shouldPaint=false. For transform-origin: 50% 50%, the originTranslate evaluates to (75, 25) (see RenderStyle::applyTransform), but to (0, 0) when using transform-origin: 0 0. The additional translation needs to be respected, otherwise one cannot judge whether the layer is painted (aka. visible) or not. Since it is now clear that neither transform nor transform-origin are taken into account, one can construct an even simpler testcase, not involving transform-origin / rotations: <div style="position: absolute; transform: translate(0px,100px); left: 100px; top: -100px; width: 150px; height: 50px; background-color: green"> This is visible but excluded from the dump, as the "untransformed" boundingBox() does not intersect the damageRect. I'd like to hear the expert opinion on the sanest way to fix this :-)
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Simon Fraser (smfr)
Comment 1 2019-10-08 11:20:48 PDT
We know that offsetFromAncestor() doesn't work across transformed ancestors, so it's wrong that writeLayers() is using this. Maybe write some code that checks for transformed ancestors and does the right thing in that case?
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