Every time I try to vote to close a question as an ‘exact duplicate’, I'm getting tired of the behaviour of the user interface. When the ‘This question is a duplicate of which other question?’ box pops up, I can give a URL or number of another question or select an already proposed duplicate.
Most of the time, especially when feeding the box a question I found myself, I already know that the answer is really the real duplicate and I don't have to wait for the contents to show up. I immediately mouse to the ‘Vote to Close’ button, but when the snippet shows up, it moves way down the screen, or even outside of the browser's viewport. I find it annoying that the target I was heading for moves around. It reminds me a bit of the classic voting machine.
One way to avoid this behaviour is to put the buttons at the top of the dialog, but of course that is silly, as most users read from top to bottom. Another solution is to give the box a fixed height, so that there is enough room to display the the proposed duplicate, but the buttons remain in the same place.