Previously, this flag wasn't cleared properly when the notebook lost
focus, but only when the notebook was focussed again later.
As this flag is only used to advance correctly focus pages by grabbing
focus to the new page after switching the page, this is not good.
In particular, it can cause a focus grab when programmatically switching
the notebook page and the focus is inside a completely different widget.

A previous attempt at this fix in
6e0af6c2 has been reverted in
dfe0c8c0 because it didn't work
correctly.