Dragging a message into composition ends up weird and not useful... (regressed from TB 91 - Alice, pls...)
Seen on 102.0a1 (2022-05-17) (64-bit), Win10.

str

• drag IMAP message (optionally with its own attachment) from 3-pane into new composition

actual

• 0 bytes attachment is added
• attachment name is Attached Message Part (missing .eml extension)
• user confused and won't know what's attached, or if there's anything attached at all
• attachment location tooltip: imap://doe@example.com@imap.gmail.com:993/fetch>UID>/INBOX>28
• saving and sending surprisingly does the correct thing (although in 3-pane, you'll see 2 attachments, the mail message and its contained attachment separately, age-old bug), but as a user, why would I try sending a 0 bytes attachment?

expected

• show correct attachment size instantly
• attachment file name should be "message subject.eml" as in TB 91
• grab the attachment at the time of attaching, as it might go away later

Here's a screencast of what happens...

Screenshot 1: Actual result, TB91 vs. TB 102.0a1
TB 91 wins!

Interestingly, Forward As > Attachment does the correct thing in 102 - maybe that codepath could help to fix the similar scenario of attaching message via drag and drop?

Regression window:
https://hg.mozilla.org/comm-central/pushloghtml?fromchange=4de0f8ef7fa6bb37048c878c8e27185610561632&tochange=dffed3eeb94166fb3d5e9ce6f329c9bc148cbf8d
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=96f5324370c76f60771abf24b4ae2da804043bf2&tochange=a8ce86cf670af93083642b33789080c463c351fe

Suspect: Bug 1766073

(In reply to Alice0775 White from comment #4)

Regression window: ... Suspect: Bug 1766073

Thumbs up & thanks!

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/8a0cae9847bb
Fix attaching a message via drag and drop. r=aleca

Verified working on 103.0a1 Daily.

Comment on attachment 9279397 [details]
Bug 1769929 - Fix attaching a message via drag and drop. r=aleca

[Approval Request Comment]
Regression caused by (bug #): bug 1766073
User impact if declined: Attaching another message from within Thunderbird via drag'n'drop doesn't work correctly.
Testing completed (on c-c, etc.): Extensive testing during review, tested on c-c, has some automated test in a separate patch
Risk to taking this patch (and alternatives if risky): Could break drag'n'drop for something else.

Comment on attachment 9279397 [details]
Bug 1769929 - Fix attaching a message via drag and drop. r=aleca

[Triage Comment]
Approved for beta

Walt, Thomas, possible for you to test the drag and drop called out in comment 9, if you haven't already?

Thunderbird 102.0b4:
https://hg.mozilla.org/releases/comm-beta/rev/9356bb71f0fe

(In reply to Wayne Mery (:wsmwk) from comment #10)

Comment on attachment 9279397 [details]
Bug 1769929 - Fix attaching a message via drag and drop. r=aleca
Walt, Thomas, possible for you to test the drag and drop called out in comment 9, if you haven't already?

VERIFIED Fixed.

I have tested all of the following on 103.0a1 (2022-06-07) (64-bit):

Tested using drag and drop onto composition:

• attach message as attachment: OK (attachment name = subject.eml, size showing correctly, also in the following) - this bug
• insert message inline: not available (by design I guess)
• attach image as attachment: OK
• insert image inline: OK
• attach txt file as attachment: OK
• insert txt file inline: not available (that's odd!)
• attach Browser URL as attachment: (OK, sort of, but... see below)
• insert Browser URL inline: OK (pastes as <a href...> full link)

Dropping browser URL as attachment behaves a bit strange (probably unrelated to this bug):

• no .html extension, may cause odd behaviour when opening attachment (Edge was once showing me the HTML source instead of rendering)
• no immediate snapshot taken (what if the page changes while I compose?)
• hence no file size shown until msg sent/saved and reopened
• I got an error when trying to open an attached html page which had a UTF8 smiley 🙂 in the web page title (and hence in the file name).

Thanks much Martin for fixing this!

Looks good to me using Thunderbird 102.0b4 on Windows 10 and Fedora 35 Linux.