Bug 297706

Summary: [TextureMapper] wrong drop-shadow position in high DPI
Product: WebKit Reporter: Fujii Hironori <fujii.hironori>
Component: PlatformAssignee: Nobody <webkit-unassigned>
Status: NEW    
Severity: Normal CC: bugs-noreply
Priority: P2 Keywords: DoNotImportToRadar
Version: WebKit Nightly Build   
Hardware: Unspecified   
OS: Unspecified   
See Also: https://bugs.webkit.org/show_bug.cgi?id=218734
Attachments:
Description Flags
test case none

Fujii Hironori
Reported 2025-08-20 18:45:12 PDT
[GTK] wrong drop-shadow position in high DPI ./Tools/Scripts/run-minibrowser --gtk --release LayoutTests/css3/filters/drop-shadow-current-color.html EXPECTED: you should see 8 square boxes ACTUAL: you see 4 rectangles GTK WebKitTestRunner works as expected.
Attachments
test case (347 bytes, text/html)
2025-08-21 04:32 PDT, Fujii Hironori 		no flags
Details
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Fujii Hironori
Comment 1 2025-08-21 02:47:47 PDT
Disabling the AC mode work around the problem. I can see 8 boxes. But, worng color, this is another bug. WEBKIT_DISABLE_COMPOSITING_MODE=1 ./Tools/Scripts/run-minibrowser --gtk --release LayoutTests/css3/filters/drop-shadow-current-color.html
Fujii Hironori
Comment 2 2025-08-21 04:32:52 PDT
Created attachment 476470 [details] test case
Fujii Hironori
Comment 3 2025-08-21 05:08:00 PDT
TextureMapper expects DIP (device independent pixel). But, GraphicsLayerCoordinated uses DP (device pixel). For example, there is 100px x 100px element in x2 device scale factor. GraphicsLayerTextureMapper and GraphicsLayerWC creates a 100x100 TextureMapperLayer. And, apply x2 scale transform to the root layer. OTOH, GraphicsLayerCoordinated creates a 200x200 TextureMapperLayer. TextureMapper::applyDropShadowFilter apply the drop-shadow filter. https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebCore/platform/graphics/texmap/TextureMapper.cpp#L1064 LayerTreeHost::applyDeviceScaleFactor and DrawingAreaWC::updateRootLayerDeviceScaleFactor apply a transform to the root layer. https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebKit/WebProcess/WebPage/CoordinatedGraphics/LayerTreeHostTextureMapper.cpp#L295 https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebKit/WebProcess/WebPage/wc/DrawingAreaWC.cpp#L309
Fujii Hironori
Comment 4 2025-08-22 05:26:24 PDT
I was wrong. GraphicsLayerCoordinated is using DIP.
Fujii Hironori
Comment 5 2025-08-23 06:03:50 PDT
I confirmed Windows port has the same issue. This is a bug of TextureMapper.
Fujii Hironori
Comment 6 2025-08-23 06:53:50 PDT
I think this is a the same problem with bug#218734.
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