Keep current line in place when math un/renders

[cben]

If I have:

foo foo
$$\frac{very high}{formula}$$
bar bar

and I move from "foo foo" into the formula and back, "bar bar" (and the rest of the doc) jumps up and down. Fair enough.

But if I move from "bar bar" up into the formula and back down, I'd prefer "bar bar" to stay at the same height on my screen and for "foo foo" (and everything above it) to scroll correspondingly.

This is very subtle and low priority but would also help during the initial rendering pass.
In that use case where widgets are created but cursor didn't move, one could argue this should become the default CodeMirror behavior? Though there are patological cases when scrolling the first line of a document off-screen would be unexpected...