I run KDE Plasma 5.9.2 in Arch Linux. I have two problems with the recently introduced global menu. (1) I made the following change: System Settings-> Application Style -> Widget Style and Behavior -> Menubar Style: Title bar button After that wmail (https://aur.archlinux.org/packages/wmail-bin/ and https://thomas101.github.io/wmail/ - version 2.1.1) constantly used 100% of my CPU. See https://dl.dropboxusercontent.com/u/2171814/Screenshot_20170214_222025.png I reported the issue here: https://github.com/Thomas101/wmail/issues/584 (2) After reverting back to "In application", Wmail no longer used 100% of my CPU. However, kate displayed no menus at all. I had to do a rm ~/.config/kate* and restart kate to get the menus back. Obs. I use the Breeze theme and icons.
See also https://github.com/electron/electron/issues/8455
As I already mentioned in https://github.com/electron/electron/issues/8455, Kai Uwe Broulik told me that the issue is on the Electron side, and that it shouldn't emit LayoutUpdated signals when nothing was actually changed. I don't feel entitled to close this issue, though, so lets wait for someone else to respond.
As for item 2), that also happened to okular. After reverting back to "In application", okular (just a kate) displayed no menus at all. I had to do a rm ~/.config/okul* and restart okular to get the menus back.
Probably you should have filed two separate bugreports for two separate issues, not one for both of them. For the item 1 — this is an Electron issue, and it has a workaround described in the linked Electron bugreport. As for the item 2, * The bad behaviour is observed when: switch to «menu button in title bar», open kate, switch to «menu inside window», close kate, open kate — no menu. * The bad behaviour is not observed when: switch to «menu button in title bar», open kate, close kate, switch to «menu inside window», open kate — the menu is present. What plays an effect here is whether the app in question was opened when the setting did change to «menu inside window», the bug is reproducable only for those apps that were open at that moment — restarting them doesn't help, the menu is still hidden. Workaround: press ctrl+m _two_ times to «hide» and show the menu.
Marking as duplicate of the other electron bug. Please file each issue invididually. 2.) should be fixed with Qt 5.7.1 *** This bug has been marked as a duplicate of bug 376476 ***