Using a window handle to grab touch points does not function correctly.

When grabbing the window handle to intercept touch points, after a WM_POINTERDOWN event has been received, no WM_POINTERDOWN, WM_POINTERUPDATE, or WM_POINTERUP events will be received for any other touch points until a WM_POINTERUP event is received for the initial touch point. 

With the same codebase, I can replicate this problem in Qt versions 5.10 and 5.12, but not in 5.9.