You have probably seen already a link, on the Featured on Meta sidebar, to an announcement of an upcoming upgrade to the text editor on Stack Exchange, Opt-in alpha test for a new Stacks editor.
As a whole, the project looks extremely useful, and it looks like it will do a lot of good for general text editing, particularly for new users who are not accustomed to writing in Markdown.
However, the current design philosophy is to get rid of the simultaneous preview of the page, which implies getting rid of the simultaneous preview of MathJax. From what I can see, the envelope of possible design choices includes some outcomes that could keep this site working, but it also includes many choices that would break this site as well as any other SE sites that depend on MathJax to function. Moreover, from what I can tell, the current trajectory is towards those outcomes that would break this site.
Thankfully, the dev team has asked for community feedback at the right (early) stage of the process, and this trajectory can be corrected. I have written up the issues I see with the development as an answer on the feedback page, and, if you agree with the issues I perceive, I would ask you to support that answer with your votes and with voicing your opinion there. The dev team has a lot of work in front of it, and MathJax will not be made a priority unless the community makes it clear, through energetic and compelling (but obviously polite and constructive) feedback, that the issues require attention and they require attention before harmful design choices become set in stone.