Reproducible with Firefox 43 beta 5, latest Developer Edition 44.0a2 and latest Nightly 45.0a1 Affected platforms: Windows 7 64-bit (2 different machines) and Mac OS X 10.11.1 Steps to reproduce: 1. Launch Firefox and go to Add-ons Manager (Ctrl/Cmd + Shift + A). 2. Click on 'Watch the Video' button via Get Add-ons tab 3. Enter fullscreen. 4. Mouse over the Fullscreen notification. Expected results: Fullscreen notification is displayed correctly. Actual results: 'Exit Full Screen (Esc)' text moves to the right and returns to its initial position (please see the attached gif). Additional notes: 1. Unable to reproduce under Ubuntu 14.04 32-bit. 2. Not reproducible with Nightly from 2015-08-15 - will investigate further.

Here is the gif - https://goo.gl/wqcRfR (couldn't manage to attach it)

I actually have been seeing this issue since the very beginning. I guess this is because of minor rendering difference when it is moved across layers.

Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. Since there is a 3s timeout to hide the fullscreen warning, it makes sense to always apply will-change transform when it is not hidden. This could also avoid slight rendering difference when it stops moving.

Comment on attachment 8691164 [details] MozReview Request: Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. https://reviewboard.mozilla.org/r/26085/#review23455

Comment on attachment 8691164 [details] MozReview Request: Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. https://reviewboard.mozilla.org/r/26085/#review23457

https://hg.mozilla.org/integration/fx-team/rev/befb8e2645ef77775bbd5ef7eb866fd51b376458 Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. r=roc

Comment on attachment 8691164 [details] MozReview Request: Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. Approval Request Comment [Feature/regressing bug #]: bug 1160023 [User impact if declined]: as described in comment 0 and shown in comment 1 [Describe test coverage new/current, TreeHerder]: n/a [Risks and why]: very low risk because it is a simple change in style, and the 'will-change' property (the only change here) is generally only a hint for the engine, and isn't meant to cause any major behavior change [String/UUID change made/needed]: n/a

Regression from 43, marking affected and tracking for 43+.

Comment on attachment 8691164 [details] MozReview Request: Bug 1227129 - Apply 'will-change: transform' to fullscreen-warning when it is shown. Fix for recent regression in fullscreen style. Please uplift to aurora and beta.

Verified fixed with latest Nightly 45.0a1, across platforms [1]. [1] Mac OS X 10.11.1, Windows 7 64-bit and Ubuntu 14.04 32-bit

Also confirming this fix with Firefox 43 beta 7 (Build ID: 20151126120800) and latest Developer Edition 44.0a2 (from 2015-11-26), under Windows 7 64-bit and Mac OS X 10.11.1