High CPU Usage under Plasma

[Aldoo]

Since Firefox was updated to version 58.0.1+build1-0ubuntu0.16.04.1 (xenial-updates), I noticed a consistent high CPU usage while Firefox was running under plasma (both by firefox and plasmashell processes).

After a little inquiry, I tend to believe this is due to a global menu protocol compatibility issue (I use Plasma 5.12/Neon with global menu bar enabled).

Facts that led me to that conclusion:
1 - Since Firefox 58 (xenial-updates, obviously patched for Unity global menu), Plasma's menu bar now displays a (surprisingly working) menu for Firefox. Previous implementations did not work at all (menus would show but not open when clicked).
2 - With current Firefox (xenial-updates), either disabling the global menu in Plasma, or disabling it in Firefox (setting ui.use_unity_menubar=false in about:config), makes CPU usage go down to normal levels.
3 - Before, I had been using Firefox 58beta (from webupd8 repository), which did not have this CPU issue... but no global menu either.

Also, FYI:
- A few months ago (until it was fixed), similar behaviour was exhibited by other pieces of software such as visual-studio-code and atom editor (both based on electron framework). They would hog a full CPU core, as long as they attempted to work with Plasma's global menu, but behave normally when global menu was disabled (either in plasma or in the incriminated editor).

This latter fact proves it is possible for a piece of software to be compatible with both global menu implementations. Current status of Firefox in Ubuntu is actually very encouraging, as the global menu is now fully working even in Plasma. There only remains this CPU issue.

Although Canonical does not officially endorse any KDE flavor of Ubuntu anymore, it would be much appreciated by Kubuntu and Neon users if this remaining little issue could be fixed!

[Stephan Diestelhorst (syon)]

Can confirm the same on Firefox 78.0.2