No cross-staff shortcut under Linux
No shortcut seems to be active for cross-staff notation under Linux.
I have tried Ctrl-Shift-arrow, Ctrl-Alt-arrow, Ctrl-Shift-Alt-arrow and Shift-Alt-arrow.
Moreover, under Edit/Preferences, I didn't find where I can define them.
Note: I use the french version of Musescore:
OS: Ubuntu 20.04 LTS
MuseScore version (64-bit): 3.2.3+dfsg1-4build1 (Ubuntu focal/amd64)
MuseScore build number not set