Hi Corsair63
Thank you for the help. You were right, it was the OpenGL settings.
If I enable OpenGL then OpenCPN slows to almost unusably slow but the windows behave better.
But if I enable OpenGL and then select "Software OpenGL" the performance returns to being pretty good, and the windows
work also.
So I guess the fix on Raspian is to enable
Software OpenGL.
In case anyone stumbles on this post in the future with the same problems.
I'm running
OpenCPN version 5.6.2-0+b75be76
Debian GNU/Linux 11 (bullseye) aarch64
Raspberry Pi 4 Model B Rev 1.4
Kernel: 5.15.84-v8+
Thanks !!