This functon is using setUpdatesEnabled(false/true) to minimize updates, but in the end the last setUpdatesEnabled(true) will call update() on the complete viewport.
This can be much slower then just repainting the old and the new selection (esp. in single selection views).

Keyboard navigation doesn't do full updates.

(found on 4.4beta1, WinXP – run any program with a QAbstractItemView with QT_FLUSH_PAINT=1 to reproduce)