I tried launching Plasma on Wayland and noticed on Kmail the message preview 
pane was black.  As I move the mouse around or click on it, the black preview 
window becomes transparent and the desktop wallpaper or other windows behind 
Kmail appear, but it can flip back to black if I continue moving the mouse.  
What I cannot see in the preview pane is the actual message content.  :-/

The content is there, as I can click on URLs contained in it, or email 
addresses - the cursor changes shape.  It's just no longer visible in the 
preview window pane, under any circumstances.

If I double click the message on the folder list it opens it up in a separate 
window, which shows the message content as it should.  I've tried resetting 
all Kmail Appearance settings to no avail.

Restarting the desktop using Xorg does NOT fix this problem.  Otherwise, both 
Plasma on Wayland and Xorg work fine - except the clipboard does not work on 
Wayland (middle click won't paste selected text on another window).

The Kmail preview window problem may be a coincidence, perhaps mesa or radeon 
drivers broke things at the latest update, but I only noticed it when I tried 
Wayland.  I recall a post in this M/L about Kmail with a similar/same problem, 
but can't find it at present.

Some questions:

Have you noticed anything similar, on Kmail or other applications, either 
running Wayland, or Xorg?

Is there some fix for this problem?

At what point will Wayland be considered the recommended way to run Linux 
desktops, instead of Xorg?

I'm thinking of holding fire before I try running Wayland on other 
installations - although I have no evidence Wayland is the cause of the Kmail 
preview pane breaking like this.