I have (these points are not nececssary to reproduce the bug, just to provide context):

1. I have two QTreeViews inside a QSplitter
2. Both tree views use the same model
3. Tree views are synchronized with each other with connect(left, &QTreeView::expanded, right, &QTreeView::expand); etc

Then (this is necessary)

1. The model is QSortFilterProxyModel
2. Source model has N columns
3. The first QTreeView displays the first M columns
4. The second QTreeView displays the last (N-M) columns (column 0 is hidden too - this is important)
5. Both QTreeViews have drag&drop enabled

The bug:

1. Drag doesn't work for child indexes in the right QTreeView (works only for top-level indexes, i.e. the ones whose parent() is invalid)
2. This happens because QAbstractItemViewPrivate::selectedDraggableIndexes ignores the child indexes. This in turn is because QTreeView::selectedIndexes() removes indexes whose parent is hidden.

       while (index.isValid() && !isIndexHidden(index))
           index = index.parent();
   

3. QSortFilterProxyModel::parent() always returns an index with column() == 0, so if the column 0 is hidden, QTreeView::selectedIndexes always treats child indexes as hidden.
4. However, the child index is visible (i.e. the user can see it on the screen) and is selectable so this behaviour is incorrect.