[Webkit-unassigned] [Bug 36366] New: Repaint loop when painting using scaled CTMs with different translations

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=36366

           Summary: Repaint loop when painting using scaled CTMs with
                    different translations
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: PC
        OS/Version: Mac OS X 10.5
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: koivisto at iki.fi


The code in RenderBoxModelScaleObserver::shouldPaintBackgroundAtLowQuality
tests if the scaling has changed from the previous value by comparing the
transformations:

    // We are scaled, but we painted already at this size, so just keep using
whatever mode we last painted with.
    if ((!contextIsScaled || data->transform() == currentTransform) &&
data->size() == size)
        return data->useLowQualityScale();

This test fails if the scale is the same but the translation changes. This can
lead to infinite repaint loop, if the document is painted in pieces using
different translations (for example for tiling).

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