[Webkit-unassigned] [Bug 138213] New: Text positioning is wrong in ComplexTextControllerCoreText if left-most glyph has non-zero glyph position offsets

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=138213

            Bug ID: 138213
           Summary: Text positioning is wrong in
                    ComplexTextControllerCoreText if left-most glyph has
                    non-zero glyph position offsets
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: behdad at google.com

As explained here:
https://code.google.com/p/chromium/issues/detail?id=352736

I said:

On the CoreText path, the issue still exists, AND it's both vertical and horizontal mispositioning.  The root of the problem is similar though: GlyphBuffer doesn't have the position of the leftmost glyph.  So fixing it would need wiring up a startPosition the way the HarfBuzz shaper does.

In fact, ComplexTextControllerCoreText.mm uses CTRunGetAdvancesPtr instead of CTRunGetPositionsPtr.  That itself has the bug.  From the CTRunGetAdvancesPtr docs:

"""Note that advances alone are not sufficient for correctly positioning glyphs in a line, as a run may have a non-identity matrix or the initial glyph in a line may have a non-zero origin; callers should consider using positions instead."""

Here's a test case, test on Safari and Firefox: http://behdad.org/urdu

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