Bug 124834 – REGRESSION(r159575): fast/block/float/clear-negative-margin-top.html reftest is failing

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKitGTK

[Zan Dobersek]

After r159575, the fast/block/float/clear-negative-margin-top.html reftest is failing with a minimal difference between the reftest and the reference file.
http://trac.webkit.org/changeset/159575
http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=fast%2Fblock%2Ffloat%2Fclear-negative-margin-top.html

Specific to the GTK port, and probably an issue in font rendering.

[Zan Dobersek]

Created attachment 217811 [details]
Simplified test case -- test file

A simplified test file.

[Zan Dobersek]

Created attachment 217812 [details]
Simplified test case -- reference file

[Zan Dobersek]

The test file draws the 'ggggggg' text over the #visible <div>.
The reference file produces a render tree in which the #visible <div> (which is now positioned relatively) is located in its own layer, and is drawn after the text, and is subsequently drawn over it.

The bottoms of the first five letters 'g' are therefor painted over when viewing the reference file.

This boils down to the text being rendered in boxes of which the height is 17px, while the font itself measures 18px in height. As a proof, adding 'margin-top: 1px' to the #visible rule simply pushes the colored <div> 1 pixel down, and the text is rendered correctly in both cases, with the <div> not being drawn over it when it is positioned relatively.

This problem is evident in basically all render tree baselines for tests that contain text -- the text is rendered 17px high, instead of 18px.

[Zan Dobersek]

Created attachment 217939 [details]
Patch

[Martin Robinson]

Comment on attachment 217939 [details]
Patch

Wow...annoying.

[Carlos Garcia Campos]

This started to pass after the freetype upgrade in r221670