This is a consequence of the hack introduced in bug 1280709. Bug 1206233 makes this more noticeable as when clicking on the 'X' icon of the camera or microphone row, both streams get stopped at once, causing the panel to flicker twice. See the discussion from bug 1206233 comment 18 to bug 1206233 comment 36.

We also have several cases where the need for calling setHeightToFit() causes issues in the Downloads Panel because there are cases where the structure of the panel changes because of update events on individual downloads. Instrumenting these cases with setHeightToFit() might cause more flickering there as well. We should probably define a special class on the <spacer> above the footer, and have the <panelmultiview> machinery look for this element on the main view. The flex attribute on this element would be set to "1" only while opening a subview, and its height or flex attribute would be reset to "0" when the main view is restored, after the animation is finished. Bonus points if we can fix the existing bug in the main menu for which the main view shrinks immediately instead of following the smooth animation of the panel and the subview.

[Tracking Requested - why for this release]: Bug 1280709 fixed the footer button issue, but introduced other height calculation issues for the Downloads Panel.

Hopefully bug 1293242 will fix this, but we should at least retest after that one is fixed.

In https://bugzilla.mozilla.org/show_bug.cgi?id=1206233#c48 I granted r+ to this patch. I don't think any changes need to be made to the patch and it applied fine for me on d33489bb8862. Panos, since Florian is on PTO until August 29th, do you want to make the call here about landing this?

I'll take care of landing that patch.

The last patch from bug 1206233 has fixed this for me.

[Triage] QE-Verify + to confirm behavior.

[Triage] QE-Verify + to confirm behavior.

Verified fixed FX 51.0a1 (2016-08-23) Win 7, Ubuntu 14.04, OS X 10.12