I'll find a better title later :-) Using master. 1/ start kmail, select a folder and wait until the message list displays the messages 2/ Open a couple large folders in tabs (middle click...) As long as the tabs are not fully loaded/populated, actions on the already loaded ones have no effect, eg: messages are not marked as read, deleting has no effect, etc...
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.