Putting a spoiler on MathJax equations does not work well sometimes, as reported herehere (1st answer).
For example,
$$a=1$$ $$b=2$$ $$c=3$$
can be written using
> $$a=1$$
> $$b=2$$
> $$c=3$$
or
> $$a=1$$ $$b=2$$ $$c=3$$
However, writing
>! $$a=1$$ $$b=2$$ $$c=3$$
or
>! $$a=1$$
>! $$b=2$$
>! $$c=3$$
results in no line breaks in the preview pane. To see this in action, edit this post and look at this block:
$$a=1$$ $$b=2$$ $$c=3$$
Where have the line breaks gone?