One of the editor's container elements has the mw-parser-output class. While that makes a lot of sense when editing on visual mode, applying it to source editing mode may cause leakage of styles which are meant to be applied to Wikitext output, not to source code view.
This may become especially problematic when using syntax highlighting, as styling may ruin the alignment between the invisible textarea element and the visible p elements, making it almost impossible to edit. Example: https://community.fandom.com/f/p/4400000000003199281
I would suggest removing that class when the wikitext mode is active. ~~~~