I'm new to Affinity and came across this thread as I too want to have a copy of the original image side by side with the image I'm working on, as I've always done in PS. This is not the same as using the history slider in layers to move through process changes, nor making a duplicate saved copy. I use PS and hope now to use Affinity (which in most respects looks great BTW) to process high resolution planetary images, when attention to very fine detail is required and IMHO there is no substitute for seeing side by side versions in much the same way as it is possible in the various adjustment tools (but without the vertical slider), especially when the changes are very subtle. The best solution I have found so far is to make a new copy (copy-->new from snapshot), then float the 2 images....
This is similar to what I'm after, but not as good, so is there any reason why a simple duplicate / copy of the original image in the same window is not possible?