Thanks for your follow-up.
1. Windows 11 Pro 22631.3447.
2. Problem happens with Chrome, Edge & Brave.
3. Yes - Windows key brings up Start11 start panel & taskbar (but seems this fixes the problem until the next reboot. Hmmm!)
4. Happens with "hide taskbar" checked either in Start 11 or Windows 11.
Intriguing that just pressing the Windows key seems to fix it. I made a simple "restart Explorer.bat" file and this fixes it too - until the next reboot.
JBM