How do I split a stack of items?

I have a stack of medpacs, and I'd like to trade a handful of them to another group member, but for the life of me I can't figure out how to split the stack.

How do I split a stack of items?

Solution 1:

Shift and left click the item stack to bring up this dialog:

If you've started typing into chat then this will leave the item linked in your chat window, in addition to splitting the stack, because the UI is terrible that way.

To avoid the chat linking you can Shift and right click the item instead, and then drag the mouse off of the item before releasing the mouse button. That last quirk is somewhat mind boggling, and makes it easy to miss.