I don't want to make this about hamburger menu vs menu bar preference, so leaving that aside, the lack of menu bar means (at least as I far as I understand) no globalmenu support, nothing is exported over dbus so globalmenu has nothing to work with, KSysGuard does have a menubar and works with globalmenu. Aditionally, programs like https://github.com/Zren/plasma-hud depend on the same interface as globalmenu, so people who'd rather rely more on keyboard (either due to outright preference or perhaps mobility issues) can't use it with systemmonitor. Now, just to be clear, I'm not asking for outright support for globalmenu or plasma-hud, but for a menubar, which should (as far as I understand) bring globalmenu and plasma-hud support "for free".
Exporting a global menubar seems reasonable. Elisa does this despite not having an option for an in-window menubar.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-systemmonitor/-/merge_requests/151
Git commit 2eb9a0f72870874d8827a7d97e336e161b6160cc by Felipe Kinoshita. Committed on 12/07/2021 at 17:53. Pushed by fhek into branch 'master'. Export global menu A +91 -0 src/GlobalMenu.qml * M +5 -0 src/main.qml M +1 -0 src/systemmonitor.qrc The files marked with a * at the end have a non valid license. Please read: https://community.kde.org/Policies/Licensing_Policy and use the headers which are listed at that page. https://invent.kde.org/plasma/plasma-systemmonitor/commit/2eb9a0f72870874d8827a7d97e336e161b6160cc