Investigate issues with tileset editor collaboration

According to a user in the discord:

> looks like any time save it knocks her out of editing the tileset etc

The report names no files or tests. Start by reproducing a collaborative tileset edit while saving in the Godot editor, and record when the collaborator is removed from editing; done means saving no longer interrupts the tileset editing session and the behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.