xenops does not "work" until buffer is (re)fontified

[dankessler]

Steps to reproduce:

First, create a simple test.tex file with content

\documentclass{article}
\begin{document}
Here is math: $x^2$.
\end{document}

1. In a fresh emacs session, open test.tex
2. Turn on xenops-mode: M-x xenops-mode
3. Put cursor inside the math: $x^2$
4. Move cursor outside the math
5. Nothing happens (expected: math would render)

However, the following works as expected

1. In a fresh emacs session, open test.tex
2. Turn on xenops-mode: M-x xenops-mode RET
3. M-x font-lock-update RET
4. Put cursor inside the math: $x^2$
5. Move cursor outside the math
6. Yay! The math renders!

Step 3 can be replaced with many alternatives that indirectly will cause the buffer to be refontified, e.g., saving, editing, etc.

As far as I can tell, this is because during activation, xenops-mode adds some font-lock keywords that should trigger rendering on cursor moving using cursor-sensor-mode, but it doesn't actually cause these new font-lock keywords to be "applied."

I imagine this can be fixed by explicitly calling font-lock-fontify-buffer when mode is activated.

This is easy enough to work around, but it caused a lot of confusion for me when I followed the tutorial, which at one point suggests that the user open a file, turn on xenops-mode, move the cursor in and out of math, and see a render result.