Set -webkit-font-smoothing: none in fast/regions/autoheight-break-after.html so reftest matches exactly in Mountain Lion
Created attachment 192981 [details]
This fixes the immediate issue with this test, but I definitely wouldn't mind a more general solution.
Comment on attachment 192981 [details]
I think we want to understand why this is different.
Created attachment 192987 [details]
Comment on attachment 192987 [details]
Committed r145749: <http://trac.webkit.org/changeset/145749>
Font smoothing seems to be taken into account only when painting the glyphs (Font::drawGlyphs).
The flow thread is clipping the graphics context around the overflow rectangle so the smoothed pixels are not painted. In the reference file this is not happening unless overflow: hidden is specified.
Why the fail is happening only on Mountain Lion? It seems glyphs can be both antialiased and smoothed (subpixel AA?).
Skia's Font::drawGlyphs implementation states:
if (!shouldUseSmoothing() || isRunningLayoutTest())
shouldSmoothFonts = false;
This doesn't disable AA. Probably some agresive AA on Chromium Mountain Lion is the culprit for the failure.
Antialiasing and font smoothing are both disabled if NoSmoothing is specified on the FontDescription and might explain why this is fixed when setting the property.