Regression: Shift select broken in QAbstractItemView

At some point between 5.9 and 5.14, Shift selection got severely broken for all item views.

Click an item in an item view. Now press Shift, and press left mouse (don't release) to select another item. All items in between are selected. Ok.

Now, move the mouse, even by just one pixel.

In 5.9, nothing happens. Ok.

In 5.14, the selection is cleared and only the item under mouse stays selected.

This bug is kind of severe, because for users high-resolution mice and monitors, or with not-super-stready hands, clicking without moving the mouse by a single pixel can be very difficult.