If you right-click an item on your Toolbar, it will split the stack in your inventory like so (this does not happen when right-clicking the item directly in the inventory):
Now do the same thing with a full inventory (you need to use an item in the toolbar which, when split into a half stack, will not fit into another partial stack in the inventory, as it would just merge into that stack). Let's try right-clicking the transport belt in the toolbar:
Whoops, the extra stack created by the hand slot pushed out the item that's been in the last inventory slot (in our case the Iron Plates):
The behaviour of the Hand Slot in this regard *might* be intended, but pushing out items of your inventory presumably is an unwanted side effect.
I added a save file in the attachments for simple reproduction; just right-click the transport belts in the toolbar.
