OK! I've checked and you have definitely found a serious bug for embedded forums.
I had misread your message in my earlier response and had only tested REPLY and got a screen like this:
All was working as I expected, but realising my mistake I have now tested an edit post screen and got this:
It's lost the "responsiveness" features, showing a rather shrunken edit box and toolbar - it didn't look like your image - but the message is editable.
Then I realised my tests had been on on a non-embedded forum. So I tried one and got this (with the keyboard deliberately removed to show a little more of the screen):
It still has no responsiveness features but now not only is the forum displayed much larger IT IS NOT SCROLLABLE to reach the off-screen parts of the text. As you suggest, this effectively makes the message non-editable. (Again, as you suggest, the edit box nicely fills the width of the screen when in landscape orientation but again, as you say, it only leaves two lines of editable text visible, making the business of doing anything impractical, if not impossible.)
It seems the responsiveness features only work as intended for New Topics and Replies. They need to be implemented for Edits as well.
Would you care to edit your initial post to include BUG: at the start of the subject line, so it is a little more noticeable to the Nabble Support team!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.