is the defaul display for a 27" iMac meant to be 5120×2880?

[Ember1205]

Many of the new boot managers will leverage your "higher-than-VGA" display resolution to give you a cleaner, crisper screen at boot time to choose your startup environment. Apparently, this one has an issue with possibly not completely releasing the memory registers or something and causing the display to not function properly when being driven by Mac OS X.

 

[milomak]

It's a boot menu to setup dual booting between OS X and Linux or Windows. Not recommended as it can cause problems not only with normal operation but as the OP found out, it sometimes can change the display resolution. However, when it's working OK, it shows a nice boot menu and easy switching between operating systems.

This is another case where the OP wasn't telling us the complete story about his problem....

i've used refind extensively over a number of years and it had never affected me in this way. so i had no reason to believe it was the cause of the problem.

 

[milomak]

from the refind fella

I have a problem where refind doesn't load the native resolution for my
Mac 4k (5120 x 2880).

It loads it as 3840 x 2160.

How can I get it to load the native resolution?

I don't know. I've seen similar problem reports from others, but I have
no hardware of this type on which to test and debug. I believe it's
fundamentally an OS X driver problem, but obviously, rEFInd does
something to trigger this bug; I just have no idea what it might be, I'm
afraid.