Expected Behavior
Show cursor markers of participants in the editor.
Actual Behavior
The cursor marker of the participant which has left still shown.

Steps to Reproduce
It shows sometimes
- open a file as userA,
- open a file as userB,
- put the two users' cursor at the same position in each editor (type sth.)
- close/leave one user's editor
- open the same project file
- extra cursor marker appears at the previous cursor position
