Suppose I write $$ 1+2+3=6 $$ and so there are profound consequences....
Before today, once the $\TeX$ was rendered, I could keep typing and still see the displayed $\TeX$ as I typed. But today it's behaving differently. I can wait for the $\TeX$ to get rendered, but then as soon as I type one character I just see unrendered code, and then I have to stop typing and wait for it to get rendered again.
What's going on?