User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:2.0b12pre) Gecko/20110212 Firefox/4.0b12pre Build Identifier: See attachment for recorded bug behavior Windows 7 Aero ON Direct 2D OFF Direct Write OFF GPU Accelerated Widows 0/1 or 1/1 D3D9 (acceleration force on/off) Reproducible: Always Steps to Reproduce: Step to reproduce: -Set tabs on bottom setting -Open as many tabs as overflow buttons are visible -Open tab that contain flash plugin that could be covered by right overflow button like: http://www.liveleak.com/mp53/player.swf?config=http://www.liveleak.com/mp53/player_config.php?token=07b_1284580365%26embed=1 Or: Reproducible also on Youtube, open Index.html from attachment (or resize browser window on any Youtube video). Actual Results: White rectangle is visible Expected Results: No white rectangle visible Work: Mozilla/5.0 (Windows NT 6.1; rv:2.0b12pre) Gecko/20110209 Firefox/4.0b12pre http://hg.mozilla.org/mozilla-central/rev/fd0817e454fe Fail: Mozilla/5.0 (Windows NT 6.1; rv:2.0b12pre) Gecko/20110210 Firefox/4.0b12pre http://hg.mozilla.org/mozilla-central/rev/199cb6282554 Changeset: http://hg.mozilla.org/mozilla-central/pushloghtml?startdate=fd0817e454fe&enddate=199cb6282554

Confirming this is reproducible also on XP. I have hardware acceleration disabled.

Proper changeset (I made mistake in Description) : http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=fd0817e454fe&tochange=199cb6282554

Regression probably after landing Bug 631698 - 1px border visible below active app tabs when there is tab overflow (using Aero Glass with DirectWrite disabled)