Problem has been described in the forum: https://wald.intevation.org/forum/message.php?msg_id=5724

I use a notebook with a screen resolution of 1024x600 pixels, so your kleopatra->notepad window is wider than my screen. When you open the kleopatra window you can resize, but when you then open the notepad window you can't make narrower. Then when you close, nor the kleopatra window can't be narrower. Vertical size is OK. Kleopatra needs to be exited to return to resizable window.

I'll have to look into the layout some more about how I can improve that.
I might check for the resolution and on smaller resolutions make the notepad show either the text area or the recipients, instead of both.

That you can't resize the window when closing the notepad should be easily fixable.

I want to fix that before the 3.1.0 release