Steps to reproduce: 1. Open Chrome or Firefox. 2. Open System Settings (with the browser still visible) 3. Close System Settings. 4. The menus for System Settings remain in the global menu bar.
I tried with kate (KDE_NO_GLOBAL_MENU=1) and some Java application and it works fine for me. (Firefox and Chrome support global menu here)
Matthew, are you on X or wayland?
I am on X. Global menus don't work at all on Wayland, nor really, for me, does Plasma. Too many crashes. But Arch doesn't provide debug symbols so I can't help with that. :( I am able to reproduce using the KDE_NO_GLOBAL_MENU=1 env variable, with kate. I'll attach a screen recording; hopefully that will help.
Created attachment 107823 [details] Recording of problem
Can't reproduce in: Plasma: 5.11.5 Apps: 17.12.1 Qt: 5.10.0 Frameworks: 5.42.0 Kernel: 4.14.14-1-MANJARO
This should be fixed with the port to libtaskmanager. Please reopen if it's not.