Partial rollback of CodeMirror commit de92ca1bab49a2f18bd6f7e9d8200a9d75270304
(between 3.13 and 3.14) to hide cursor/selection bugs with display math.
TODO: isolate example & report.
Full rollback of CM to before problematic point -
styling `.CodeMirror-widget { display: inline-block; }` still left
issues with cursor movement, display math was hard to enter.
Size editor to almost fill the page.