[MU4 Issue] Problems with undocking/docking sidebar panels

Describe the bug

The drag behaviour of side panels appears to be very buggy and unpredictable. By comparison, MS3 drag behaviour is consistent.

To Reproduce

Display 2 or 3 panels in the sidebar (using the F7 / F8 / F9 shortcuts).

Note the following:

• If 2 panels are sharing in the sidebar, you can drag the left-hand one to the right, but you can't drag the right-hand one to the left. If you do it will push the other tab downwards.
• If 2 panels are sharing, it seems impossible to stack them one above the other.
• If 3 panels are sharing, if you attempt to drag one tab to the left or right, then the other two panels are pushed downwards. If you try to drag the panel down it will end up pushing the other panels down instead.
• if there are two panels sharing a position and one panel below, you cannot drag the solitary panel upwards to make it share. with the two others.
• This is only a small sample of the errors possible.

Expected behavior

Drag behaviour should be strictly consistent, allowing panels to share a position (displaying tabs) or to be stacked.

Platform information
OS: Linux Mint 20.1, Arch.: x86_64, MuseScore version (64-bit): 4.0.0-2667920094, revision: github-musescore-musescore-0f1e7fb