Comment 18 for bug 124315

Metacity doesn't do a very good job of minimising overlap - it places windows firstly into any empty space which is big enough for the entire window, and if there isn't a space big enough to do that, it places the window at top-left regardless of where the largest remaining space is on the screen. Sawfish used to work the way you're thinking of, placing things into the biggest remaining empty space even if they didn't quite fit there. I do agree placement behaviour should be configurable though.