If you click to expand or collapse a tree item but do not release the mouse button, QTreeView continues to process mouse move events as if you had clicked on an item. That is particularly problematic if your widget implements drag & drop:

• select an item
• click to collapse another
• move the mouse a bit before releasing the mouse button

=> QTreeView drops the selected item into the item you just collapsed.

I am using Qt 5.5.1 at the moment, but the code in 5.8.0 seems to be the same.