Minor changes to the session timeout window were implemented in
MDL-34498. This issue is just for the UX team to review those changes and implement any further modifications they request.
The current solution (on 3.8) is:
When the user has not interacted with the page for 9/10 of the session timeout (default 2 hours) they will get an alert window with an option to extend the session or cancel. If the session does timeout they will get a window telling them it has expired.
Both these windows can be dismissed which gives the user to copy any unsaved data from the page.