[Interest] Why can't I sort QTreeView?

[Matthew Woehlke]

Okay, this is driving me nuts...

I have a QTreeView. The model is set to a subclass of
QAbstractSortFilterProxyModel, which is in turn proxying a subclass of
QAbstractItemModel.

I've set sortingEnabled, but for the life of me, I can't get sorting to
actually work (under user control); no sorting indicator is shown, and
clicking on the column headers does nothing.

What am I missing?

-- 
Matthew