User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:86.0) Gecko/20100101 Firefox/86.0

Steps to reproduce:

On Gentoo Linux I compiled mail-client/thunderbird-78.9.0 and executed on the commandline
xdg-open "mailto:jstein@gentoo.org?cc=jstein@gentoo.org&reply-to=www@gentoo.org&subject=TEST"

Actual results:

BCC and CC are not visible, becaue the scrolling in the GUI has several glitches.
The scrollbar does not appear, when it should. The minimum size of the upper part is to small. See video (using the mouse wheel to scroll in the video).

Expected results:

The TO, CC and BCC lines should be permanently visible as it worked earlier.

The filled To/Cc (from mailto:) are shown for me (tested on trunk).

I see the exact same behavior. It's clear it was intentional, but it make for a lousy user experience. What's more, Reply All behavior has now people who WERE in the CC line shifting to the TO line, which means an observer with the message can't say on his or her own who was being replied to.

This is a somewhat of an edge case as I suspect you're experiencing the compose header area not growing to accommodate the CC and BCC field because your compose window is not tall enough.
We should improve the layout of the message header and implement some smarter handling of multiple fields/recipients to avoid header scrolling, which indeed is not great UX.

(In reply to Alessandro Castellani [:aleca] from comment #3)

Not for me. I can stretch the down the body to nothingness and still not see a CC field. I'll add an attachment in a moment to demonstrate.

See previous comment

Sorry, I explained myself incorrectly.

The CC and BCC fields are not visible by default. You can open them by clicking on the Cc or Bcc buttons to the right of the identity field.
When clicking those, the header area should expand to accommodate those, does it happen?

You can also always show those fields by default by changing the settings in your account.

• Right click on mail account name folder in Folder Pane and select 'Settings'
• Select 'Copies & Folders'
• Select checkbox: 'Cc these email addresses: '
• Leave the field empty.
• Close Account Settings tab and reopen the compose window.

Thanks Alessandro, but Thunderbird (78.13.0 (64-bit), Linux) refuses to leave the fields empty: I can empty them, move away to another settings section, come back to "Copies & Folders" and it shows my own e-mail address in these fields, which is obviously not the point.

I understand it's a balancing act between ease of use/clean interface and flexibility, but for anyone working with a lot of e-mail, having to use a mouse hurts quite a lot. I wouldn't mind one bit if the setting to permanently turn on CC and BCC fields was buried in some obscure, under-the-hood setting as long as I can reach it, but it's impossible (by design, as far as I can tell) and that's a problem.

Sorry, my mistake.
The feature I reported in comment 6 is present in 91.

No problem, thanks for letting me know this issue is already resolved ("Allow showing empty CC/BCC rows in compose window"), just waiting for an upgrade path!

(In reply to Jonas Stein from comment #0)

On Gentoo Linux I compiled mail-client/thunderbird-78.9.0 and executed on the commandline
xdg-open "mailto:to@example.com?cc=cc@example.com&reply-to=reply-to@example.com&subject=TEST"

mailto:to@example.com?cc=cc@example.com&reply-to=reply-to@example.com&subject=TEST

I've tested the original link with 91.9.0 (64-bit), Win10, and that's wfm, too.

Further improvements (e.g. preventing comment 5) will ship in 102 from Bug 1720348 - Dropping pill on CC or BCC buttons causes header area to have the wrong height (with reduced window height): Improve addressing area auto-height algorithm, fix splitter behaviour

Expected results:
The TO, CC and BCC lines should be permanently visible as it worked earlier.

They have never been permanently visible earlier.
You can now make them permanently visible via account settings (empty auto-cc/bcc) using comment 6.
In addition, you can always easily open up and/or focus CC/BCC fields using Ctrl+Shift+C or Ctrl+Shift+B. Try it!