When I type braces in math mode in a post, like $\{1,2,3\}$
, they don't show up in the preview. See the following screen shot.
I tried all the different Math Renderer options, and disabling Fast Preview, with the same results.
This affects the main MO site, but not Meta.MO (so don't try to test it on this post).
When I submit the post, the braces show up correctly. Likewise, existing posts seem to render fine, but if I try to edit them, I don't see braces in the preview.
Typing $\\{1,2,3\\}$
makes the braces appear in the preview, but I expect this would mess up the output in the actual post.
I'm pretty sure this just started recently, as I think I would have noticed it before.
I can reproduce this with Firefox 42.0 on Ubuntu and Safari 9.0 on iOS (using the Full Site).