To traverse messages (i.e selecting next or previous message) in a folder (the message list) Kmail uses PgUp and PgDown keys or right or left arrow keys as default. I think it is unituitive since people are used to traverse list boxes with up/down arrow keys. I know It's configurable but it's frustrating to find correct keys and cumbersome to edit shortcuts. So I suggest changing the defaults as up and down. Also changing the current message causes message list to lose its focus and it is not efficient while skimming messages quickly. Reproducible: Always Steps to Reproduce: Open any folder and try to traverse the messages
I agree on that it's confusing. But in general being able to scroll through the message list and the messages without the need of refocusing is a good feature. There should be only some sort of information at the start to inform the user about this possibility. I did use KMail for more than a year now and only with your description of the problem I learned that you could use right/left keys for navigating the message list. I did ignore the problem until now and clicked with my mouse, which is less efficient. So thanks for that. ;) Loss of focus is not a problem if you use separate keys for traversing the message list and scrolling the messages.
FWIW Apple Mail solves this by using up/dn arrow keys to navigate the message list, but bubbles pgup/pgdn and space/shift+space to the message itself. So you can use the arrow keys to navigate through the messages, but can still scroll within the selected message without having to leave the keyboard