Used to be when you selected something to buy from a merchant the quantity window would pop up and already have the focus. Then you could simply backspace twice to change 100 into 1. Now that window doesn't automatically get the focus so you have to find it on-screen and click in the quantity field before you can change the amount. Not a game changer but certainly a change to many years of functionality.
Also, this window can be hiding behind the inventory window (annoyingly!) and you are left wondering why you cannnot buy or do something.