That's a bug I've already noticed long ago, but since I am now reporting bugs, I realized I might as well talk about this :

When holding an item (so that it follows your mouse), if you hold the "Pick Block" key, it's supposed to create a full stack of that item in any empty slot the cursor hovers over. This is what happens with the default key (i.e. "Mouse 3", the wheel button), but it seems to truly be the ONLY key that works for that. For other keys, it simply does nothing.

I personally changed that key for something on the keyboard because I don't like to bind something to the wheel button (it's easy to roll a little the wheel at the same time), but I know there are also people that just don't have a wheel button (since pushing on it is used to switch between notched and smooth scroll).