Since the upgrade to the latest version, the compose window behaves strange. "Compose messages in HTML format" is disabled.

Steps to reproduce the bug:
1. Reply to a mail with some text paragraphs.
2. Click at the end of some quoted text paragraph and press Return to respond to that paragraph.
3. Type some characters, e.g. "Why?".
4. Use Backspace to delete what you have typed.
5. For every character deleted, a newline is added at the end of the mail before the signature text.
6. When you delete the first character of the line, the line disapeares completely and the cursor is moved after the end of the quoted text.

I can reproduce the bug with a completely fresh Thunderbird profile where I only added my mail account and disabled "Compose messages in HTML format". No other setting changed.

Hello,

the bug still exists with the new 140.4 package, and also on Debian 13 Trixie.

Please note that the bug also occurs if you did not configure a signature. It's only
not so easy to see as the "added lines" do not move the signature footer further down.

Please report if you need any missing information!

Best regards,

I can report, that the problem seems to be solved with thunderbird
1:140.5.0esr-1~deb12u1 is fixed. I did not check the Trixie version yet and will
report when I tested the Trixie version.

Unfortunately, there is still a small issue left (or newly added):

When you delete the first character of the line, a new line is added below the
current cursor position, but at least, the cursor is not moved after the end of the
quoted text.

This behavior is still unpleasant but much less annoying than the original buggy
behavior.

Can anyone confirm this behavior with thunderbird 140.5.0esr-1 for Trixie or
Bookworm? Or is this the intended behavior?

the issue still exists in Thunderbird 1:140.6.0esr-1~deb13u1 and I
assume for Bookworm as well.

Regards,