timmmay:

 

Swapping the desktop and laptop between HDMI and DisplayPort seems to have helped. I had them this way around originally but swapped them because of the same issue. If I make sure only one device is turned on at a time it seems to be manageable. If it causes more problems I'll get an HDMI switch, though finding one with EDID that passes through monitor information makes it significantly more expensive.

 

My last three monitors have been Dell, but I think this is the last one I'll buy.

 

I managed to finally solve this by switching to this cable.

I figured I'd try one that is certified by VESA, and it's first one listed when you search for ugreen: https://www.displayport.org/product-category/cables-adaptors/?ps=ugreen It's pretty amazing though just how many manufacturers aren't VESA certified. There's no certification listing for almost any of the brands commonly stocked by New Zealand retailers. Cruxtec, Dynamix, Digitus, Unitek, Promate, alogic, Vention, Baseus - none of them have any cables listed on DisplayPort.org as VESA certified. The only two manufacturers I found that have VESA certified products were Ugreen and Startech.com.