Closing a file using the tab bar "X", makes it hang on quit
This is on current master: When we have multiple files open and I close one using the "X" on the Notebook tab, everything seems fine. However when I then quit the app using the Window close button (perhaps also Ctrl-Q?), it will hang infinitely and be non-responding.
Ctrl-C then leads to a stacktrace saying something about being in threading.lock_acquire()...