I briefly tried elph180, g7x and sx710 built from the branch r5182.
On the digic 6 cams, I see the same horizontally doubled edge overlay srsa_4c mentioned, didn't notice anything else wrong in a quick look.
The main difference is I don't have to use the 14x32 font system because the bitmap is 720x480 so there are enough characters to display the CHDK menu.
G7X and the EOS M cameras have a 720x480 displays, and work fine (or at least acceptably well) with the existing code. It may be complicated and ugly, but the code is already there.
If G5X has the same underlying display, I really don't understand what benefit would outweigh the ongoing maintenance impact of adding yet another display system.
This avoids all the complication and allows the display code to work like older cameras (pretending to be a 360x240 display).
Given that the other code already exists and has to be maintained for the 640x480 D6 cams, what is the practical benefit? I guess grids and some games would be at the right scale (+/- aspect ratio), but it seems like that could be addressed in much lower impact ways.
I appreciate that you've put a lot of work into this, but my gut reaction is really strongly against having one port do things in a wildly different way without a significant underlying difference in the camera.
If there's a better way to handle this I'm happy to discuss it - I thought a separate branch for now would be simpler to manage.
Having a separate branch to test is totally reasonable, no objection to that.