Boot Problem with External Monitor on Mint 19.3

Here's an odd problem that's driving me insane.

I have a laptop (Lenovo Z370) which is connected to an external BENQ monitor.  The laptop screen resolution is set to 1366x768 and the BENQ to 1920x1080.  This works fine.

However, I installed a kernel upgrade, yesterday, and this may be giving issues with my Brave browser (refused to load and had to be re-installed).

I decided to revert the kernel from 415...121 back to ...118.  There is normally an option in GRUB to choose an alternative kernel and boot from it.  The offending kernel can then be removed, later.

However, although the GRUB menu appears on the BENQ monitor (set as primary) I cannot scroll through it and select an alternative boot kernel.

If I physically disconnect the BENQ then GRUB appears on the laptop and works normally.

If I make the laptop the primary display then GRUB appears on that but still doesn't work.

I'm using Mint 19.3 with the Cinnamon desktop.

Any thoughts?