I'll attach two test cases to this bug to demonstrate the issue. I encountered this when I was working on bug 389321. I'm going to disable the respective reftest <http://mxr.mozilla.org/mozilla-central/source/layout/reftests/bugs/503531-1.html> in that bug for now. When this bug is fixed, that reftest should be re-enabled.
Created attachment 486269 [details] Incorrect case This is what the frame tree for this test case looks like: http://pastebin.mozilla.org/827730 Note that the y position of the empty text frame is 1152. This is wrong.
Created attachment 486270 [details] Correct case Here's what the frame tree for this test case looks like: http://pastebin.mozilla.org/827732 Note that the y value for the textframe which contains \u200c (ZWNJ) is 1248, which is the correct value.
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