Somehow the "drop" event is firing even though it shouldn't, and that causes cards to be added to the destination book then deleted from the source book. If they're the same book then the card is deleted.

I think the cause is something to do with a "dragover" handler outside the address book browser, or maybe on the browser itself. That might be confusing the code into thinking there is a valid drop target when there isn't. It's hard to tell.

Assuming this affects 102. If not, please update the flags. Thanks

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/f64404889698
Prevent drop action on address book deleting the dropped card. r=thunderbird-reviewers,freaktechnik

Comment on attachment 9280222 [details]
Bug 1773249 - Prevent drop action on address book deleting the dropped card. r=#thunderbird-reviewers

[Approval Request Comment]
Regression caused by (bug #): new AB
User impact if declined: (small) potential for data loss
Testing completed (on c-c, etc.): landed last week
Risk to taking this patch (and alternatives if risky): low

Comment on attachment 9280222 [details]
Bug 1773249 - Prevent drop action on address book deleting the dropped card. r=#thunderbird-reviewers

[Triage Comment]
Approved for beta

Thunderbird 102.0b5:
https://hg.mozilla.org/releases/comm-beta/rev/f40b84f044ed