Comment Haiku's been doing this (Stack & Tile) (Score 1) 38
Haiku (the BeOS reimplementation as FLOSS) has been doing this for some years, building on BeOS' tabbed title bars (which one could slide with shift-click but that's about it), and implements "stack and tile": you can dock windows by their sides, and stack them so them are grouped, then you can resize them as a whole and move them too. This was a contribution of someone from the Auckland University.