Steps to reproduce:

• Have an account where signing and attachment of the key is enabled by default
• Compose a new message
• Disable signing of the message (which also disables the key being attached)

Actual result:
The attachment pane is still shown, with a label of "1 Attachment (0 bytes)" and no content.

Expected result:
The attachment pane disappears.

This is with Thunderbird 91.0b6

But we attach the key only at send time, so I don't understand why there was an attachment in the compose window to begin with?

Maybe crucial missing step then: sending fails because the external openPGP can't use the key it's asked to (smartcard not available). So I then disable signing, which leads to this.

So probably on a failed sending we remove the attached pgp key, and that removal doesn't properly trigger the update of the bucket UI.
I'm not too familiar with the PGP code handling the attached key, if you can point me to those methods I can see if I can take of fixing this.

Looks like its removeAttachedKey() here: https://searchfox.org/comm-central/rev/2372e390f3b6add0f2b3530964472fa0e2e4463c/mail/extensions/openpgp/content/ui/enigmailMsgComposeOverlay.js#878

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/3e92677b485f
Ensure the attachment pane is updated when GPG keys are added or removed. r=mkmelin

Comment on attachment 9236234 [details]
Bug 1725447 - Ensure the attachment pane is updated when GPG keys are added or removed. r=mkmelin

[Approval Request Comment]
Regression caused by (bug #): bug 1695521
User impact if declined: When a PGP encrypted email fails during sending, the attachment pane remains visible with a wrong count number, as if the attached PGP key was still there.
Testing completed (on c-c, etc.): on c-c
Risk to taking this patch (and alternatives if risky): low

Comment on attachment 9236234 [details]
Bug 1725447 - Ensure the attachment pane is updated when GPG keys are added or removed. r=mkmelin

[Triage Comment]
Approved for beta

Thunderbird 92.0b3:
https://hg.mozilla.org/releases/comm-beta/rev/4bb9883a4755

Comment on attachment 9236234 [details]
Bug 1725447 - Ensure the attachment pane is updated when GPG keys are added or removed. r=mkmelin

[Approval Request Comment]
Regression caused by (bug #): bug 1695521
User impact if declined: When a PGP encrypted email fails during sending, the attachment pane remains visible with a wrong count number, as if the attached PGP key was still there.
Testing completed (on c-c, etc.): on c-c and beta
Risk to taking this patch (and alternatives if risky): low

Comment on attachment 9236234 [details]
Bug 1725447 - Ensure the attachment pane is updated when GPG keys are added or removed. r=mkmelin

[Triage Comment]
Approved for esr91

Thunderbird 91.0.3:
https://hg.mozilla.org/releases/comm-esr91/rev/060c4a346efa