Yeah, I
posted about this issue about two months time back, and as mfb has said, it is due to the lazy-loading of MathJax. The workaround is to do the following:
Case 1: If you are replying to a thread that does not have LaTeX before your post in the page:
1. Write some random LaTeX in the reply box, like
##a## test.
2. Make sure the draft is saved (green dot adjacent to floppy icon; or save it by clicking on the floppy icon → Save draft).
3. Refresh the page.
4. Now, hit preview. LaTeX will be rendered normally. Delete what you had written earlier and start typing what you want.
Case 2: If you are creating a new thread:
1. LaTeX will obviously not be loaded. Write some random LaTeX code.
2. Save the draft.
3. Refresh the page.
4. Now, you will see that LaTeX has rendered in the message box. Delete the rendered LaTeX and start typing what you want. Preview will now work.
Note that in case 2, if you type something and then refresh the page, or save the draft, leave the page and come back later, LaTeX will always get rendered in the box, so whatever you have written will become gibberish, especially if you have fractions, integrals, differentiation operators, etc. This is a bug. If you are starting a new thread, it is probably better to write it out somewhere else and then copy and paste here.
Maybe Greg can keep the LaTeX to load lazily, but design a button somewhere on each page that will be displayed only if the MathJax resource is not loaded on that page. Clicking that button will load LaTeX for that user, and it will stay until the user refreshes the page.
