Currently, the edit merge screen follows a two-column layout. This makes sense for big enough screens. When the screen becomes too small, though, the boxes need to be sorted below each other. Please find out, when they should be put below each other and implement the correct behavior for too small screens, and window resizing.
This is part of fulfilling wish #1 of the German-speaking community wishlist 2015 (T139601).
The solution is a result of on-wiki discussion and Wikimania 2016 sessions and testings, as well as feedback we received to the prototype.