Public bug reported:

I recently swapped out my WUXGA monitor for a much bigger 4K monitor.
Most unexpectedly, I found that this prevented my workstation from booting.

When I start up, I see the power-on splash screen from the workstation
motherboard.  I should next see the Grub menu, but instead I get a
blank screen, and the boot-up hangs indefinitely.

If I unplug my monitor for the first few seconds of a power-up, until
the Grub menu has come and gone, I can re-plug the monitor and watch
the rest of the boot-up into the initramfs and my system.

If I use my old, small monitor, boot-up works.

I solved the problem by using efibootmgr to change the boot loader
from /EFI/ubuntu/shimx64.efi to /EFI/ubuntu/grubx64.efi.  With this
change, the system boots properly with the new monitor: I see power-on
splash screen, Grub menu, initramfs messages, etc.

For all the above, Secure Boot is enabled by my firmware.
Another way I can fix the boot hang is to turn off Secure Boot.
Then my workstation boots normally with shimx64.efi.
(It is counter-intuitive to me that Shim is the boot loader blocked
by Secure Boot, but that's what I observe.)

To summarize: of the 3 variables I explored, the only combination
that fails is Secure Boot, a 4K monitor, and shimx64.efi.

It seems someone else had the same problem with a new monitor in 2016:

I am running Ubuntu 18.04.4 ("Bionic").
My motherboard is an ASUS P10S WS, firmware version 3801.
grubx64.efi is from grub-efi-amd64-signed version 1.93.16+2.02-2ubuntu8.15
shimx64.efi is from shim-signed version 
I also have shim version 15+1533136590.3beb971-0ubuntu1 installed.

I am happy to run experiments on my hardware for you.

** Affects: shim-signed (Ubuntu)
     Importance: Undecided
         Status: New

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  Shim bootloader hangs before Grub menu with 4K monitor

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