[Webkit-unassigned] [Bug 124021] New: Layer contentsScale can differ between first paint and repaints

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Thu Nov 7 16:17:47 PST 2013


https://bugs.webkit.org/show_bug.cgi?id=124021

           Summary: Layer contentsScale can differ between first paint and
                    repaints
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: thorton at apple.com
                CC: simon.fraser at apple.com, dino at apple.com


Steps to Reproduce:

1. Open the attached test case.
2. Cmd-A.
3. Cmd-R.
4. Goto 2.

Expected:

The text should not shift.

Actual:

The text shifts.

It's pretty imperceptible, but if you do some logging, you can see that the layer has "root-relative scale factor"=1.411765 during the initial paint and "root-relative scale factor"=1.2 in subsequent paints.

More logging shows that the transform that maxScaleFromTransform decomposes is:

[1.20 0.00 0.00 0.00]
[0.00 1.20 0.00 0.00]
[-0.82 -0.35 1.20 -0.00]
[-93.83 -46.95 120.00 0.85]

in the bad case, and:

[1.20 0.00 0.00 0.00]
[0.00 1.20 0.00 0.00]
[0.00 0.00 1.20 0.00]
[-20.00 -20.00 120.00 1.00]

subsequently. Dean thinks it's odd that m44 is non-1 in the bad case, but it's also bad that this changes between paints, because it means things shift (and could be blurry in the initial paint, if drawn too large and scaled down awkwardly).

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