When dragging a list of emails to an external program (an Electron app in this case), Thunderbird only provides information (a URI) for the first email in the list. I wrote an extension to work around this, but with the deprecation of XUL overlays this no longer appears to be an option.
In the external program I’m catching drop event data, by order of preference [“text/uri-list”, “text/x-moz-url”, “text/x-moz-message”]. My recollection is that these all produce basically the same content—something of the form: “imap://email@host:143/fetch%3EUID%3E/INBOX%3E6062”. The data only ever indicates a single UID, regardless how many emails are selected. The extension I wrote for Thunderbird modifies the produced string, using commas to add additional UIDs within the same folder: “imap://email@host:143/fetch%3EUID%3E/INBOX%3E6062,6063,6067”.
Our external program has greatly facilitated our sorting of email. At this point we’re hoping that the underlying issue can be corrected in Thunderbird, otherwise we’re left wondering where to go from here. The non-XUL-overlay extension options don’t seem to allow for this sort of flexibility.
I note also that the extension “Disable DragAndDrop” will likely also be affected. That extension has also saved me numerous headaches. There’s a bug on bugzilla requesting the behaviour from around a decade ago, further highlighting the importance of powerful extensions like provided by XUL overlays.
I already sank a day into figuring out the new legacy behaviour (?!), but got nowhere. I’m not keen on sinking more time into a path that’s already slated for deprecation. At this point I’m mostly considering ways to move away from Thunderbird, and just hoping that the release isn’t as disastrous to us as the beta.