When I want to move windows to the side of the active desktop (for putting two of them on the same screen) some programs are able to fit to the side (i.e. they resize accordingly), some only do that for specific configurations (f.ex. texmaker does that only if the internal pdf viewer is closed) and some (f.ex. the document viewer) simply does not change size when trying to "glue" it to the sides. Why? And how can I change that behaviour if I want to have it for all programs?
the program may be setting a minimum width above the width of your screen / 2. Gala will refuse to tile or fullscreen apps that won't fit. can you manually scale the window so it would fit there? if you can scale it file a bug against gala.