1515331 - A flicker is displayed while changing between the "Default" and "Dark" themes

Status: NEW

(Firefox :: Theme, defect, P5)

Description

[Emil Ghitta, QA [:emilghitta]]

[Affected versions]:
Firefox 64.0 (BuildId:20181206201918).
Firefox 65.0b5 (BuildId:20181217180946).
Firefox 66.0a1 (BuildId:20181217220100).

[Unaffected versions]:
Firefox 60.4.0esr (BuildId:20181203164059).

[Affected platforms]:
Windows 10 64bit
macOS 10.14.
Ubuntu 18.04 64bit.

[Steps to reproduce]:
1.Launch Firefox.
2.Click the “Hamburger” menu.
3.Click the “Customize” button.
4.Click the “Themes” button.
5.Click the “Dark” theme.

[Expected result]:
No visual issues are displayed after switching between themes.

[Actual result]:
A flicker can be seen on the URL area.

[Regression range]:
I have encountered 2 behaviors while searching for the regression. 

There is a more discrete flicker on the URL bar that was introduced by one of the bugs found in this generated pushlog: 
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=9b20ccf5448b55feef48f9830268b7acf53eac75&tochange=6bb7337970f19723f9ba34a682b4682c53863e5a

The more visible flicker (shown in the provided screencast) was introduced by the bug found in this generated pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=e732689cf7e797420d24b5a7c4248100ecb3c63e&tochange=469e083ccd18793bec63a26b3ffd6dd47d78c810

[Additional information]:
For further information regarding this issue, access the following link (in order to view the screencast): https://drive.google.com/open?id=1WtTQqNgEE8ICadeb6cmsaYFaD3IgVFx_