[benrbray]

Normally there are two types of math: inline and block. Inline math is delimited with single dollar signs like $x+y=5$, while display math uses two dollar signs with newlines, like

    $$
    \int_a^b f'(x) dx = f(b)-f(a)
    $$

You can find some GIFs of the ideal behavior here [1], but it's understandable that such a thing might not be a priority for most users.

The next best thing would definitely be a popup! A natural toggle button would be $, though it may make more sense to only insert math on a quick double-tap of $, so users can still write about money :). Assigning it to CTRL+4 or similar might also be a good compromise.

[1]: https://github.com/benrbray/prosemirror-math