User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

1. gfx.webrender.all = true in about:config
2. Under Linux/Gnome/Wayland/AMD graphics, launch: MOZ_ENABLE_WAYLAND=1 firefox
3. Left click on the toolbar button of an installed extension.
4. A popup displays below the button that should allow you to change extension options.

Actual results:

The displayed popup is usually completely blank, only the border and background are displayed.
Clicking multiple times on an extension that displays a small area popup means it sometimes renders OK.
You can still click on the popup area and invoke extension functions, it's just invisible.

Under Webrender/X11 (without MOZ_ENABLE_WAYLAND) the popup displays but if you mouseover the Omnibox the popup can be corrupted visually.

Expected results:

Popups function normally with content visible like in Firefox 82.

This bug has also been reported upstream in Fedora:
https://bugzilla.redhat.com/show_bug.cgi?id=1899331

I am also able to recreate this issue on Fedora Linux 33, Firefox 83.0-8 (Fedora build), GPU Intel(R) HD Graphics 520 (SKL GT2).
I'd been using Firefox with WebRender enabled on Fedora Linux for quite a while - at least from FF 78 through to 82 - before encountering this issue.

To reproduce

1. Ensure gfx.webrender.all = true is set in about:config
2. Launch Firefox with Wayland window
3. Have at least one third-party extension installed and enabled from addons.mozilla.org (I have reproduced with uBlock Origin, Kee - Password Manager, and Simple Tab Groups)
4. Ensure that the third-party extension has its button visible in the Toolbar
5. Left click on the extension button
6. Pop-up area appears below the button with the extension's content area

Result: Pop-up content area is blank. Border and background match that of the active theme. It is possible to interact with the content (eg. clicking where buttons should be visible triggers the correct action).

Expected: Pop-up content for the given extension is visible.

Workarounds:

1. Disable WebRender. OR,
2. Leave WebRender enabled, but pin the third-party toolbar buttons to the Overflow menu. Left-clicking the button in the Overflow menu list correctly displays the extension's content.

Notes:

• This issue does not happen with any of the built-in toolbar buttons (eg. Downloads, Firefox Account)
• I do not see any pertinent errors reported in the Browser Console.

Same issue here with Debian Testing GNOME-Wayland, and Firefox Debian package manually set to Wayland + WebRender.
If I deactivate WebRender, the bug is "solved"

Confirming the bug on firefox/arch(83.0-2)/wayland/iris. I ran mozregression to identify the commit that caused the issue:

20:43.35 INFO: Narrowed integration regression window from [b5017b0a, 2165f99d] (3 builds) to [b5017b0a, 604e2561] (2 builds) (~1 steps left)
20:43.35 INFO: No more integration revisions, bisection finished.
20:43.35 INFO: Last good revision: b5017b0a0b5d10f00957b15fa9081d3f755dad76
20:43.35 INFO: First bad revision: 604e2561311f2aa2ba70fbccdfb54127506ad8b5
20:43.35 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=b5017b0a0b5d10f00957b15fa9081d3f755dad76&tochange=604e2561311f2aa2ba70fbccdfb54127506ad8b5

The commit in question is:
604e2561311f2aa2ba70fbccdfb54127506ad8b5 - Sean Feng - Bug 1669239 - Call Tick to paint in PuppetWidget::Paint instead of using WillPaintWindow r=emilio,mattwoodrow

Sorry about introducing the regression.

I believe this is going to be fixed in 84.