retsync plugin makes IDA hang when closing IDA

This error has been around for a long time and is very annoying, causing IDA to hang.
Open an IDA database file, work with it, then close it while still not exiting IDA
Then open another IDA database file, work with it, close it, then exit IDA.
IDA won't close the following database file and hangs.
I checked and there is a thread in IDA plugin still waiting.
Tks

Reproduce the sequence in IDA: open and close one database, open and close a second, then exit IDA. Inspect the retsync IDA plugin's thread and shutdown handling, focusing on the thread reported as still waiting. Done means the second database closes and IDA exits without hanging.

Written by the indexing model from the issue text.