User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9b5) Gecko/2008032620 Firefox/3.0b5 Build Identifier: Thunderbird 2.0.0.14 (20080421) In "Grouped by thread" view, a message that is deleted and then undeleted is not grouped back into its thread. Reproducible: Always Steps to Reproduce: 1. Set a message folder's View to "Grouped by thread" 2. Delete a message from that folder that is grouped together with other messages 3. Edit|Undo|Delete Actual Results: Message appears as a separate item in the folder Expected Results: Message should appear grouped with messages that belong to the same thread

I get slightly different behavior that's a little less buggy. If I delete a message in the thread and then undo the delete, then then restored message shows up at the end of the thread list for the thread it belongs to regardless of where it should really be placed in the thread. The View->Sort by menu is Date, Ascending, Threaded. View->Threads is All. This was with both Tb 2.0.0.14 and 3.0a1. btw, there is no "Grouped by thread" menu item in the View menu.

Bug 93923 might be related. Confirming this on trunk though.