STR: (Win7_64, Nightly 43, 32bit, ID 20150913030204, new profile, safe mode) 1. Set layout.css.devPixelsPerPx -> 1.25 OR set layout.css.devPixelsPerPx -> -1.0 (default) and set font size = 125% on your OS 2. Open enough new tabs to cause overflow in TabsToolbar Result: Shadows in TabsToolbar overlap navigation toolbar by 1px. I see this issue ever since good old HiDPI mechanism was replaced by brand new one. Expectations: Shadows in TabsToolbar shouldn't cross the TabsToolbar rectangle (visually) Note: Browser toolbox says that #nav-bar has border-top-width (as well as border-left-width and border-right-width) equal to 0.8px instead of 1px, like in normal mode [layout.css.devPixelsPerPx -> 1.0 (100%)]. Maybe that's what causing this, but I'm not Mozilla employee, so I can't say for sure. If I set that pref to 1.2 or 1.7, border-top-width is fractional, but if that pref is set to 2.0, border-top-width equals to 1px.

Um, I think that it's not actually a regression, because old HiDPI didn't have shadows. Maybe I incorrectly expressed it: Shadows were implemented in Fx32 and they behave like this all this time

This bug was "implemented" between 2014-06-10 and 2014-06-11. Pushlog_url: > https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=9dc0ffca10f4&tochange=18c21532848a Implemented in bug 878020. However, I think the root of the problem is bug 477157