This is likely something you could customise up with CSS using media queries. The problem you’ll run into, though, is that users will get a very different interaction depending on the device they use and how wide their browser window happens to be. Especially on macOS, users tend to not maximise/fullscreen their windows all the time, and instead shuffle and resize them arbitrarily, so they might go to respond to a topic and get the “vertical” composer, then five minutes later get the normal one.
Maybe it would be worthwhile exploring making this an option on the editor’s “grippie” – kind of how browser developer tools work. This would then make it an explicit user action, so there would be fewer surprises.
I agree that composing long replies can be irksome – you have the binary choice of either a nice big composer or the ability to see the post. So I admit that I will occasionally use multiple browser windows (and screens, if I’m at the office) to “fake” the behaviour you’ve outlined, @mrjn.