Auto scroll Discord preview with editor

[Towsti]

Auto scroll (scroll-sync) the discord preview with the editor.
Ie the preview scrolls to the information that is visible in the editor.

I've experimented with this quite a bit and also tried out some existing implementation.
None of the third-party scroll sync implementations I could find are suitable because they're all using a relative scroll sync.
This eventually misses information, especially for things like images and embeds where the height of the preview is very different from the editor.

What seems like the best solution is to assign the line as a div id for the Discord preview.
The editor can use cm.getViewport() to get the visible lines in the editor. The Discord preview then scrolls to the div with a ID that matches the top visible line.