Affected versions

• 82.0a1 (BuildId:20200825094622)
• 81.0b2 (BuildId:20200825191644)

Affected platforms

• Ubuntu 20.04 64bit

Steps to reproduce

1. Launch Firefox.
2. Open any webpage.
3. Hit CTRL + P.
4. Open the pages dropdown.

Expected result

• The dropdown is correctly displayed.

Actual result

• Intermittently, the dropdown is overlapped by the preview section.

Regression Window

• Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=80e6be24260d34c7638715107e676806817d0a43&tochange=89bbbe5e6369e1ce2b75ddf75cd201782ce362b2
• Potential regressor: Bug 1653389

Additional Information

• I'd set this S4 since it's a glitch that reproduces highly intermittent (maybe 5 tries out out 100).
• I was unable to reproduce this on Windows 10x64 and macOS 10.15.
• No console error are present.

Hi Cornel,

we've made a bunch of changes in that code recently. Is this still reproducable on Nightly?

This video from the duped bug looks like it might be helpful here https://drive.google.com/file/d/1s8CzdYaB1vR7B1ab8dZgs0ezRWGwWxgP/view.

I'm still able to reproduce this with 2020-09-10 Nightly (buildID 20200910152110).
Removing the regressionwindow-wanted flag since it was provided in the description.

Moving to 83.

Cornel, can you check if you have WebRender enabled? If so does toggling it change anything?

It seems this issue is related to the WebRender indeed. It does not reproduce after force disabling the WebRender.

Cool, I'll take it out of the print2020 list, then. :)

Gnome Xwayland, Debian Testing, Intel HD Graphics 630 (KBL GT2)
The dropdown is a WebRender window.
mozregression --launch 20200825094622 --pref gfx.webrender.all:true gfx.webrender.debug.new-frame-indicator:true -a https://example.com

Also noticed: The drowndown is entirely frozen for a second until the highlight background is removed from the dropdown button, you can't hover (highlight) the other entry in this period.