There's a "hole" in my KMail window!

For the last few Fedora and KMail releases I've had a "hole" in my KMail 
window as shown in the screen-capture at this link: 

    https://postimg.cc/image/9yc897pmf/

The hole picks up whatever was on the screen before it or under it, is always 
in the same place and no others, and moves with the window. Re-sizing any of 
the 3 panes does not move it.

I can't find any way to clear it. It's fine when KMail starts but always comes 
back. No other windows exhibit this behaviour. 

Unfortunately it blocks the Unread number for the Avra folder, which I suspect 
might cause it.

My video is a nVidia GT630 using the nouveau driver.

What the heck???