Twice all the tree structure disappeared for a window, is this a feature I accidentally enable, or some bug?

[mzso]

I had this annoying event twice. All of the sudden I see all the tabs going on the top level.
It wasn't just a display issue. I could only restore the tree structure by restoring the last session from the backups.

Not sure what happens, but I don't see a hotkey for TST that does this.

[irvinm]

It does happen for some, but certainly shouldn't be frequent. ("Tabs become flatten suddenly / tree structure is lost accidentally" in the FAQ)