Default to side-by-side layout
The current default layout keeps the message view/pane below the message list.
This is terrible for screens with 1280x720, 1366x768 resolutions and pretty bad even with 1920x1080 resolution.
HTML email also tends to have extra paddings or sections at the beginning and end of the email that take a significant chunk of vertical space.
Instead, default to a side-by-side layout.
A side-by-side layout sacrifices some horizontal space for the sake of gaining a lot of vertical space. Since the loss in horizontal space is not huge and the title/description in the message list usually doesn't take that much horizontal space, the message list is not significantly affected by this at all.
When there's not enough horizontal space, plain text in the view flows to the next line, and HTML email gets a scrollbar if necessary.
Since it's a change to the defaults you can always change it back under Settings -> Configure KMail... -> Appearance -> Layout if you don't like it.