It should simply fill up the first visible free slot with the pattern that was clicked.

What is tricky about this is that the server would usually process the shift-click actions, but it doesn't know what the position of the scrollbar on the client is. This means that a client action is required in the menu, sending the desired source slot and target slot (by serial) for quick moving and letting the server sort it out that way.

Well, Duplicate of #6743 :-)