Affected versions

• 82.0a1 (20200920213416)
• 81.0 (20200917005511)
• 80.0.1 (20200831163820)
• 78.2.0esr(20200817153328)

Affected platforms

• Windows 10x64

Steps to reproduce

1. Open Firefox and go to about:addons.
2. Click Enable/Disable for a random theme (Dark or Alpenglow).

Expected result

• No visual glitches are displayed.

Actual result

• Topside of the browser shows a white flash.

Regression range

• Pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=eab754e02657af34096c2a5a79c208d393a72830&tochange=70a50fe09a18b2516e9ffdb2083debfab7de720d

Notes

• Attached a screen recording.
• The issue can be reproduced with random themes from AMO (e.g Science Fiction Florest).
• I cannot reproduce the issue with OS Light theme.

Suggested Severity: S4

Hi Emilio,
this issue seems likely to be triggered from LightweightThemeConsumer.jsm, did you notice something that may be able to trigger this (or have ideas of what we should look into) when you did optimize the LightweightThemeConsumer.jsm as part of Bug 1661123?

We'd get the right default theme notification afterwards anyways.

This prevents flashing the default light theme.

The patch I attached fixes it but not 100% sure it's the right fix, as I'm not all that familiar with this code. The issue is that when disabling a theme we get two update calls, one with theme: null, one with the actual default theme... the theme: null one flashes the default styles.

Emilio, this bug is getting into very extension-specific code. Do you want to re-assign the bug to me, or are you interested in continuing to work on this?

It seemed low priority and I've been a bit busy with TPAC and such going on. If you could take it over the finish line that'd be great, thanks!

Otherwise I'll eventually get to it I guess, but be my guest if you have the cycles :)