FWIW, I’m running RC13 HiDPI on Windows 10 (no linux involvement) and if I understand the question, I have my editor split one document on top and perhaps some other (but it makes no difference if it is the same) document in the other editor.
If I double-click on the split divider after I’ve adjusted the split so one editor window is larger than the other, nothing happens for me either. The editors remain uneven size.
I switched over to side by side split, and repeated the scenario and it made no difference.
Note: I’ve tried adjusting my clicking speed from very slow, to as fast as I can go, and many variations in between (though I have a pretty good feel for how fast a double-click with my settings is), and nothing changes.
It’s worth noting (if you ask me) that to switch between top/bottom splitting to side-by-side I first went back to no split (one document in only one editor) and then held down the ALT key when clicking on the split icon the next time.
It was then that I became aware that even though the “split screen” icon now looked like a full page—i.e., it looked like it would close split screen (which it does if you don’t hold down the ALT key)—it did allow me to directly switch from side-by-side to top/bottom splitting without having to go back to a single full editor in between.
The good news from the Hey, perhaps this is a workaround to the double-clicking not working perspective is that each time you use the icon to switch the type of split, the resulting windows now look like they are the same size.
The bad news is for anyone expecting to keep the former split ratio in the new orientation!
I think, ideally, switching from horizontal to vertical split (or vice versa) should remember user-desired split ratio—but that’s just one users desire—and, certainly, the divider double-click ought to work if that’s how it’s designed to work.
The real takeaway from my testing is: I, also, see the originally reported behavior (as I understand it) and it has nothing to do with Linux.