In our GUI we do drag-n-drop using the standard UE4 blueprint nodes.
After a drag-n-drop the next mouse click is ignored. So to drag again you must click twice.
Also after a drag-n-drop the editor buttons are all dead on the next click.
I think this is a know bug. Is it getting fixed any time soon?
I believe you are having a focus issue not a bug. Look into the “Set Keyboard Focus” and “Set User Focus” nodes. Depending on your GUI setup set one or both of these in your widget on one of the mouse move,enter,leave functions and you shouldn’t need the double click anymore. I use drag and drop for my players inventory, I use the on mouse move function to set keyboard focus to an inventory item widget and this works for me.