It happened to me too. (UE4.18.2)
Place a button in Widget Blueprint and make “On Pressed” event handler, place breakpoint on it.
While playing in editor press the button to activate the breakpoint.
after stopping PIE right click dragging doesn’t scroll my widget blueprint, but only make dotted rectangle uselessly.
(I guess it’s because the button doesn’t get mouse up event)
This is how I resolve it.
Place a breakpoint on BeginPlay Event in Level Blueprint, Play in editor and it activates breakpoint and then stop PIE. right click dragging goes back to normal.
I hope it to be fixed soon.