Thank you everyone for your patience on this
I had previously tried to change the default composition mode but didn’t find the setting – total failure on my part to be sure, in searching the settings. Anyway, I’ve now found it, and changed it, and I’m getting the markdown editor with preview again, which is awesome.
Even better, now my math works!
FWIW I didn’t know what the toggle looked like or where to find it, so I didn’t know how to switch it manually. Thanks @mcmcclur especially for that screenshot.
On the component, I think it is enabled, but for some reason not working, but now I’m not even sure it matters much to me. Am I right that this is just for hiding the mode toggle? I’m not sure I actually care whether or not people can toggle the mode, as long as it’s possible to make the math work, and especially since it will work in the default mode.