Bug 434056

SUMMARY

When I click on menus in the menu bar of Firefox Nightly 86.0a1 (2021-1-15) - 88.0a1 (2021-3-6) and 86.0 on Wayland with WebRender enabled in a Fedora 34 KDE Plasma installation with Plasma 5.21.2 and earlier, the menu items under the cursor usually aren't highlighted. Sometimes just the first item in the menu was highlighted. Occasionally multiple items in the menu were highlighted at the same time. The menus shown when right-clicking on a page are also affected by this issue. This problem started with 86.0a1 (2021-1-15). I ran the following to look for the commit involved
MOZ_ENABLE_WAYLAND=1 mozregression --good 2021-1-13 --bad 2021-1-18

The end of the process showed
13:17.81 INFO: Narrowed integration regression window from [25680895, 8a9e98e5] (4 builds) to [376c2ff6, 8a9e98e5] (2 builds) (~1 steps left)
13:17.81 INFO: No more integration revisions, bisection finished.
13:17.81 INFO: Last good revision: 376c2ff62872fbf7cc25803f9cbdcd42f8c1fe6c
13:17.81 INFO: First bad revision: 8a9e98e50d031c7a72429a08aad7325f50e34d2e
13:17.81 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=376c2ff62872fbf7cc25803f9cbdcd42f8c1fe6c&tochange=8a9e98e50d031c7a72429a08aad7325f50e34d2e

The first bad revision was https://hg.mozilla.org/integration/autoland/rev/8a9e98e50d031c7a72429a08aad7325f50e34d2e

Bug 1686703 [Wayland] Merge GetWaylandBufferWithSwitch()/GetWaylandBufferRecent() to GetWaylandBuffer(), r=jhorak

Depends on D101746

Differential Revision: https://phabricator.services.mozilla.com/D101747

The problem doesn't affect 86.0a1-88.0a1 and 86.0 on X in Plasma on Wayland with the same Firefox settings and system versions.

STEPS TO REPRODUCE
1. Boot a Fedora 34 KDE Plasma installation updated to 2021-3-6 with the updates-testing repo enabled
2. Log in to Plasma 5.21.2 on Wayland
3. Install firefox-wayland if it isn't already with
sudo dnf install firefox-wayland
4  Start Firefox on Wayland with WebRender compositing enabled 
5. Show the menu bar if it isn't already by right-clicking on the tab area and selecting Menu bar
5. Select the Bookmarks menu
6. Move the cursor up and down over the Bookmarks menu
7. Repeat 5-6 for other menus in the menu bar
8. Go to a site such as https://bugzilla.mozilla.org
9. Right-click somewhere on the page
10. Move the cursor up and down the right-click menu

OBSERVED RESULT
Menu items under the cursor usually aren't highlighted in Firefox 86.0-88.0a1 on Wayland

EXPECTED RESULT
Menu items under the cursor would be highlighted normally 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 34
(available in About System)
KDE Plasma Version: 5.21.2
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I reported this problem at https://bugzilla.mozilla.org/show_bug.cgi?id=1693472
Vlad Zahorodnii's comments on that report seemed to indicate that the underlying problem might involve kwin_wayland. I'm unsure if the problem is in Firefox or kwin_wayland.