Cool thank you
Maybe I can help by providing some more information I gathered through debugging.
I just saw that the problem exists because the Widget doesnt rebuild itself anymore
When I close my Inventory and reopen it the Text shows the right number.
This happens after RebuildWidget is called.
Unfortunately it’s a protected function so I cannot test my supposition.
I have found a work around for this until it gets fixed (works in 4.13.2). But when you drop the widget, get the widget that is being dragged, get its class, and make a new widget from that. Then add that to the canvas/viewport and set its position.
If you have any variables which reference the widget you are moving, you are going to need a function that takes an input of the user widget, casts through all the available references you have until you find the right one, and sets the new widget to be that (and maybe does a refresh of the inventory as well for good measure).