The following layout test fails sometimes on platform GTK
When it fails the diff is the following: http://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20%28Tests%29/r171113%20%281433%29/fast/forms/select/listbox-click-on-scrollbar-pretty-diff.html
Running it in a loop until it fails should suffice to reproduce the wrong behavior.
I have marked this test on the GTK TestExpectations on r171169 <http://trac.webkit.org/r171169>
The test(s) filed under this bug have been consistently passing for the last 4000 revisions. Marking bug as fixed.
Committed r263254: <https://trac.webkit.org/changeset/263254>