@fritzs
New Quizzes is a separate program than Canvas (it's an external tool) and has a different equation editor than the Rich Content Editor, which is used extensively throughout Canvas.
MathQuill is the basic editor for the New Quizzes, not Classic Quizzes. There is no advanced view for New Quizzes, you type the content directly into the question, wrapped in \( and \) delimiters, which is awesome for those who type a lot of equations or who want to copy/paste content without having to go through and convert the content aftewards.
You are correct, though, that you cannot see what it looks like until after you save the question. That is something I would love to see improved.
Another change is that New Quizzes mathematical content is rendered using KaTeX and some things are rendered slightly differently than with Classic Quizzes (see example at bottom of this post). MathJax is used with Classic Quizzes.
For what it's worth, \overscore{AB} does not work in New Quizzes or Classic Quizzes: it should be \overline{AB} or perhaps \overline{\mathrm{AB}}
The \mathrm{} makes it use a Roman (non-italic) font in math mode. Some people also use \text{} as it's quicker to type than \mathrm{}, but it does kick you out of math mode.
Here is a rendering of \overline{AB} ~~~ \overline\mathrm{AB} with both engines.
Classic Quizzes:

New Quizzes:

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