I assume this came along with the most recent update. I use a 21:9 screen (2560x1080). Typically i will split the screen to half browser tab for apiant and half either a spreadsheet or notepad++. It appears now that the screen size is no longer supported for that.
If you send me the number of horizontal pixels you need for your ideal split I can see about supporting that in the next release. Sorry for the unexpected behavior.
so if i split 50/50 the full window would be 1280. i would assume that would work. the only thing i can think is it is looking at the internal window measurement and the scrollbar is taking away from that 1280 and that is the issue. at least that is my guess. so i would think anything over 1250-1260 would be good. i can get exact if you need it