Whenever I open a mailbox (IMAP, disconnected) in KMail/Kontact, Kontact regenerates the thread information (i.e. re-collates the threads in that mailbox). This information should be cached alongside the messages, to speed up display in Kontact. The information could even be generated right when an email is received - i.e. the email could be "inserted" directly into the thread it belongs to. Reproducible: Always
Thank you for the bug report. As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists. If this bug is no longer persisting or relevant please change the status to resolved.