Re-joining a Realtime editing session discards unsaved changes

Steps to reproduce, having realtime editing enabled:

• In a first browser window, log-in as alice and start editing a page.
• In an incognito window, log-in as bob and edit the same page.
• Back to the first browser window, make alice leave the realtime session by unchecking the checkbox.
• Make some changes in the first browser window.
• In the first browser window, join the realtime session again by checking the checkbox.

Expected result: Alice is prompted wether or not she wants her changes to be integrated in the realtime session or discarded.

Actual result: The changes made by alice are discarded and the content is replaced by the realtime session's current content.