A lot of code checks for groups being restored rather than session being restored

[ThosRTanner]

Which'd mean odd things could happen. At least one place says it's checking for session to have been restored.

[ThosRTanner]

Note: session restored is problematic as there is one session object for each window (which is in itself questionable as a session is browser wide), and only the one from the main window gets the begin/end events.
On the other hand I'm not sure how one could validly be in a state where the groups weren't valid (unless you somehow managed to create a tab between window loading and first tab being restored).