@Debenben The approach I tried, and you're more than welcome to give it a shot was to inspect the page, and make the text field visible (currently it's transparent) then tweak with the padding to get them to overlay exactly atop each other.

I too was having this issue, making the new editor unusable with color highlighting. Seems the issue is the misalignment of CodeMirror (providing syntax highlighting) and the traditional input field (which is transparent). I added this to my common.css and seems to be working well, probably a horrible way of fixing it, but seemed to do it..

Yeah, all that makes sense. Turns out IntelliJ has "Copy as plain text", which resolves the issue. Do people really expect formatting to carry over during edit source mode? Is there a convo about this? (Just curious)

Okay, so I think I found the issue, which is actually complicating things. I'm copy and pasting from an editor I built based on IntelliJ/Webstorm and the likes (WP:PUGGLE). When I paste directly from there, it wraps a bunch of no wiki tags around it, but if I paste into a flat plain text editor, and copy and paste from there, it works fine. My guess is because when you copy from IntlliJ, it keeps formatting and attempts to apply it. So maybe it's coming across as richtext vs. plain text? Here's an example.. https://en.wikipedia.org/wiki/User:Drewmutt/sandbox