If a QSortFilterProxyModel is used in a QTreeView, the view may collapse the items if a certain number of items are filtered out in the same sub-hierarchy at once.

I.e. if the tree looks like this

• root
• item 1.1
• item 1.2
  -item 1.3
  -item 2.1
  -item 3.1

and items 1.1, 1.2 and 1.3 are expanded and then subsequently filtered out at once by using filterAcceptsRow(), then the view will collapse all items, while still showing a "-" sign indicating that there is a subtree present that's collapsable.