Left click on folders and messages in either folder pane or message list becomes unresponsive, the last viewed message is frozen in the message pane. Drop down menu items seem to work, eg view-message-list-sorting-aggregation can be selected and box ticked. Minimizing the window and then restoring will return Kmail to normal operation most of the time. Rarely, I would have to quit and restart kmail. This fault occurs on an average of once every twelve hours of continuous usage. First noticed last year with openSUSE leap 42.2 and is now manifest with leap 42.3.
Thank you for reporting this issue in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version? If you can reproduce the issue, please change the status to "REPORTED" when replying. Thank you!
This problem has not manifested itself following upgrades to version 5.5.2. However, I cannot add much in regard to recent versions since kmail2 was dropped as my mail client at the start of year 2019. This ticket can be closed.