I am having the same issue when launching Stardew Valley from Steam, and not only does the window appear non-fullscreen and moves down and off screen completely on its own, like the other users' problems, my initial big picture setup no longer works as it once did with launching the game. As well as the window drift, the window comes up on my second monitor (not the one that steam's big picture mode is activated on) and is not full screen. I have brought up other games on steam with the same exact setup since this issue started and the launch is still done properly, i.e. they come up on the first monitor where big picture mode is activated and automatically in fullscreen and therefore this is an issue specific to Stardew Valley.
Stardew Valley's launch worked fine like my other games initially for the first few weeks. If it helps the context in which this problem started happening was when there was an emergency alarm going off at my apartment complex and I had to close my laptop while Stardew Valley was up and running (not just at the title screen but actively playing the game) to take it with me while I evacuated the building, so the problem could be due to a bad shut-down (even though when I opened my laptop again the game was fine). I only noticed the problem after I loaded the game up for a subsequent time. I have tried relaunching the game multiple times, shutting down and relaunching Steam multiple times, and restarting my laptop. I also do not have any mods and I just got the game a few weeks ago, and while the game is playable this is frustrating to deal with.
