The case is with inserting an extra new line (visually) when the container is shrink to nearly at the end of a line. Here are the cases I've found......
Shrinking the container nearly the end of the first line doesn't cause an extra new line to appear between the first and the second one.
Seems if the second line is longer and is already wrapped while reaching the end of the first one there's no extra blank space inserted. I'm applying the HTML file test case I've used below.
Nice comments. Would you add your testcase and these notes to bug 25680? http://bugzilla.mozilla.org/show_bug.cgi?id=25680
~fantasai
