Your use case

Not exactly a bug but kind of a design defect.

The problem

Say I am in Element Web. I want to send a private message to user @Susan@example.com with username Susan. I type,
/msg Susan Test message

I get this

Okay, so I should have typed Susan's full name, or tabbed to autocomplete a full name. Fine.

But, when I click OK, the /msg text I had entered is lost completely! Hitting the up arrow to access my previously sent messages, like on IRC, goes only to the last sent message, not the mistyped last entered message. If I had written a long message, I would have lost the text completely and it would have been very frustrating.

Expected behavior

Clicking "Ok" should restore the (mistyped) /msg text in its entirety in the entry field.

If this is for some reason impractical, the error field should have a button for "Copy entered text" or "Restore entered text".

Have you considered any alternatives?

Because nobody types wrong syntax on purpose, there is no way to plan ahead for this and therefore no real alternative.

Additional context

Tested this today with https://app.element.io