Error in a4a7af4732.
To allow deleting tree elements while iterating, the new iterators would
get needed data out of the tree element before calling the iterator
callback. This included the info if the element is open or collapsed. So
if the callback would open or collapse elements, the iterator wouldn't
respect that change. Luckily the way the open/collapsed state is stored,
we can still query it after the callback is executed, without having to
access the (possibly freed) tree element.
43ddfdb1a5