Musescore 4: cannot split staff like in MS 3

Hello,

In Musescore 4, I cannot split staff like in MS 3 (no context menu entry available).
I tried either of the following methods:
a) selecting the range of notes by selecting the first one, then Shift+selecting the last one. No entry in context menu for splitting staves.
b) selecting the first note, then "select similar on this stave". Selection works, but trying to call the context menu deselects the notes immediately. No entry in context menu for splitting staves.

This all works in Musescore 3.6.2
(see https://musescore.org/en/node/125121)

Guenter