Dragging items with the middle mouse button causes block count to go negative

Screenshot attached.... Hard to explain but very easy to repo:
1) click normally on a item/stack and go to place it into a workbench
2) hold MiddleMouseButton and drag the stack around the workbench. Stacks of 64 will appear all the while the count of items in your 'held hand' will in fact go into 'negatives' (and turn red in the # text) while the oddball and magical stacks of 64 are yellow.