Created attachment 395690 [details]
If you go to http://diveintohtml5.org/, you'll notice horizontal scrollbars in the table of content. This is a regression from bug 498428.
More specifically, it comes from the non-upstream patch http://mxr.mozilla.org/mozilla-central/source/gfx/cairo/win32-cleartype-clipping.patch introduced in bug 445087.
Apparently, it only happens when using a web font (lin-libertine-min.ttf in the testcase).
This also happens with the default font in the bugzilla comment box if the length of the line is just right and in the facebook chat text entry box where it is quite annoying because the box is only one line high and the scrollbar blocks the text.
The test case does not show a scroll bar on linux and related bugs are all fixed. Assuming fixed, please re-open if not.