Changing spacer length with a mouse is not undoable
1) Create a score with more than one staff.
2) Add a spacer.
3) Save/close/reopen (or just open test_spacer.mscz).
4) Double-click a spacer, drag its end to change its length.
5) Try to undo (Ctrl+Z).
Result: nothing happens, length change doesn't get reverted.