MarkdownEditorView was listening to some frequently-emitted signals on

CodeMirror, performing calculations, and then emitting its own signals.

This ended up taking enough time that it led to a slowdown of text

input, making the input feel sluggish.
We now throttle the events. Viewport changes are limited to one every

250ms, and change events are limited to one every 500ms.
The result is that the widget went from sometimes sluggish performance

to near-native (though this will likely still depend on how overloaded

the browser is and what the page is doing).

Tried typing in the fields, and saw that the text wasn't feeling as

sluggish rendering on the page.
We'll have to see if others feel that this is really making an

improvement.