Cross-staff notation

In piano scores, it is common to write a musical phrase extending across both staves—bass and treble. This can be entered in MuseScore as follows:

To start with, enter the notation in one staff. e.g.
Select a note and press Ctrl+Shift+↓/↑ (Mac: Cmd+Shift+↓/↑). This moves the selected note, and all other notes in the same voice, down (or up) to the other staff, e.g.

Note: It follows that if you want to ensure that notes remain on the same staff during the operation, they should be in a different voice to the one selected above. Be aware that notes which have been moved to a different staff still belong logically to the staff of origin. This is why no rest is shown in the place where the moved notes originated.
To adjust the beam, double-click it to show the handles. Use the keyboard arrows or drag the handles to change the beam angle and height: