User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:94.0) Gecko/20100101 Firefox/94.0

Steps to reproduce:

Open a new compose window with cc option enabled or replay to a mail with cc address.
move cursor to below formatting bar so that it is displayed as resize cursor.
single tap/click when resize cursor is there.

Actual results:

Address area collapse, Visible only 'To' address bar.
There is no scroll bar visible. If we resize address little bit, scrollbar re-appear.

Expected results:

Address bars should not collapsed.

I can reproduce. It might be expected behavior.

However, if you do it, then attempt again after expanding the addressing area it does not collapse

Hi Jaise, thanks for pointing this out!

(In reply to Wayne Mery (:wsmwk) from comment #1)

I can reproduce.

Me too. Weird. See attached screencast.

It might be expected behavior.

Not really, and it's not consistent as Wayne said. Collapsing would only be acceptable on double-click imo. Also, if by design, expanding should work in the same way, but does not.

However, if you do it, then attempt again after expanding the addressing area it does not collapse

The whole algorithm for calculating the height of the addressing area isn't in good shape.
Henry is working on that in bug 1720348, I'll set a dependency (maybe it can get fixed there). It's complicated because there's many different scenarios to consider.

I'll ask Henry to keep an eye on this in bug 1720348.

The whole algorithm for calculating the height of the addressing area isn't in good shape.

Yeah, this whole section needs a better layout and approach in general.

Fixed in bug 1720348