(Edit: I should note that I was not monitoring indexing via the Activity Manager before I enabled indexing on the couple of additional folders, so I can't know whether this issue is new after having done so, or whether it's existed forever.)

Based on your last topic, have the email in those account fully synced as yet? Indexing will be shaky at best if those email are still downloading.

Do you have an antivirus installed tat might be scanning modified files in you profile folder? It is very common and the root cause of a lot of indexing failures. Thunderbird does not share well, so create an exception in your antivirus program for Thunderbird entire profile folder to prevent issues caused by scanning etc. As you would be aware the global massage index is quite large and is also accessed frequently, so antivirus interference is often first noticeable on that file specifically, followed by settings not saving between sessions and issues with the inbox.

Unfortunately neither of these are it; the account is fully synced (it really is a very low volume of messages with little data), and I don’t have any AV running.

It’s strange, it will be hanged (as it is now), I can quit and reopen the application and it will index a few more messages and hang, and I can repeat. I suppose if I do that enough times it will actually eventually index all the messages, but that’s clearly not a solution.

Any other thoughts much appreciated.