Highlighting multiple emails render them together.

You can see the fourth email is loaded on-top of the first email, from the screenshot, but they are actually all rendered in that view.

Viewing multiple emails in the message view only works well for plain text or simple HTML emails. Some HTML emails use various kinds of CSS styling which doesn't work well when “concatenated” (HTML was not designed for this purpose. The only quick solution would be for me to disable this feature when viewing HTML emails, but this would be bad because it often does work well. The other solution is for me to completely reimplement the message view to actually be multiple message views. I might do this for other reasons, but it would also have the side-effect of making it impossible to select text spanning multiple emails.

I'm sorry I don't have a better answer for you.

(Thinking out loud: Another option might be to “fix” the HTML to be better suited for “concatenation”, but this is non-trivial.)

I was honestly more thinking just not rendering them at all when multiple emails are selected, I guess that's what I expected and was surprised at. If that could become a setting of sorts I think I'd prefer that actually. Sorry I somehow never got an email when you replied and basically forgot about this ticket.