Though I have long grown accustomed to the vertical layout of the "Ask Question" app. I realize now, that it is probably a great bug to request be fixed. You notice, right, when you're working on a math question, that editing past 50 or so lines becomes cumbersome? Right, I have to keep scrolling up and down, up and down, and the MathJax content (I'm guessing) makes it so that this up/down scrolling is not even fluid (useable). The end result is that this bug is identified as a major bottleneck for any mathematical pursuit involving MSE. One pseudo-solution I've found is to zoom out the page, but that of course is limited by your content length (when it grows huge) too.
I have some credibility when it comes to UI design. I have written a complicated 60+ Python 3 file PyQt5 app. And maybe 5 other tiny PyQt5 apps. I am also a 10+ year user of web pages, like most of us, so can judge what is a good design and what is not. Also, this is not some crazy futuristic feature request, but actually something the site engineers could, ideally, whip together in a few weeks.
The simplest solution is to allow a checkbox option on the Ask Question page (for desktop sessions only; not mobile) that toggles between vertical and horizontal views. Vertical viewing is what we already have. Horizontal viewing is putting the text edit box on the left, and the MathJax post previewer on the right, so that on a standard laptop, the user can view both sides no mater how long their post becomes. Additionally, the side-by-side (horizontal) viewing auto-scrolls to your current place of editing. This results in less needless stress for every actively participating MSE user!
The current MO of the Ask Question app in regard to long posts is very troublesome to me, so I have also tagged this post with "bug", as it is by definition a UI design bug. Please give your support for a better-working MSE site by voting this up and commenting.