Created attachment 256707 [details]
Open the attached test case and compare the first and the second paragraphs.
Result: The second paragraph is shaped in two different passes as if they are separate runs. So the last character of the first run has a "final" shape and the first character of the second run has an "initial" shape.
This bug is similar to https://bugs.webkit.org/show_bug.cgi?id=6148 but this one is way more important. It hits millions of Facebook users very often when part of the text is hidden and its visibility is controlled by "See more..." link. Once the link is clicked, the other span is visible and the bug is reproducible.
NOTE: It is reproducible in Chrome bug fixed in FireFox.
*** This bug has been marked as a duplicate of bug 6148 ***