Cannot select a mail subject to copy-paste

"The long term plan for the headers view is to retire it and instead show this information in the message view itself."

Do I read this right that you are planning on including the headers in the message itself so that it scrolls away with the body of the message? The attachment should explain what I mean. Let me know if you'd like to have a separate ticket for this "feature request".

Yes, that is what I mean. The main reason to retire it is actually that
it'll make it much more flexible with regard to showing multiple
messages in the message view. (Note that you can select multiple
messages, but it is with quite primitive headers shown which is why it
is not yet a proper alternative to the explicit headers view.)

The problem is that the fixed (i.e. non-scrolling) header (particularly in a non-collapsed state) takes up a lot of vertical space.

I got that. And the future change (no promises on when) will also make
the headers scroll out of view.