I've marked D18867 as fixing this. It isn't a perfect fix, but it's closer to the realm of reasonable (you have a sensible amount of area to work with and a readable font):

In an ideal world we'd render a full remarkup control (so you could typeahead emoji, drag and drop files, etc) but since this control needs to render completely on the client and the full remarkup control has a significant server-side rendering component today, it's a lot of work. This is surmountable, but I suspect there are few bulk edit use cases for adding comments with tons of rich detail to many tasks, and thus that it isn't a ton of value.

If you're about to do this (make a huge, complex comment via bulk edit), I think you often might want to put the big rich block of text somewhere else and just link to it in the comment (e.g., "Closing obsolete old tasks, see Txxxx for discussion of why we're doing this.") so that you can edit/update the rich text and users know where to discuss it if they have a question.

You can also always compose a rich comment in another editor and copy/paste it, in the worst case.

I imagine we'll update this control (and the similar control in Herald) some day after some stronger use case for making it easier to render this control on the client side arises.