If you are holding an item and you left-click and hold on an inventory slot, no progress bar appears, but if you continue to hold the mouse button long enough the contents of the slot disappear. It's as if you had picked up the whole stack, but because your hand was full the items were simply removed from the slot and not put anywhere. This happens whether or not the slot contains the same item as the one you're holding.

Also, if you hold briefly but release the mouse button before the progress bar would have been full, no items are lost but the stack you're holding isn't added to the slot despite the slot containing a partial stack of the same item.