XnView Software

How about add an option to maintain high quality zoom-IN beyond screen size?

XnView 1.90 <x>

Do you mean, some thing that turns HQ off when it's larger than the screen, but keeps it on when it's smaller or fit to the screen? Because your request is worded in such a way that it seems you want HQ to stay on with high zooms, which is the current behavior. (It helps to be very clear about what you're asking for the first time.)

atm, hq zoom turn off when image is larger than screen

Wrong. Here's how to test: Zoom to 500% in view mode, then enable or disable HQ in the options. Switch back and forth if you can't see the difference at first, but I assure you, it's glaringly obvious. It's also much slower in HQ mode.

If you're seeing a bunch of jagged dots on diagonals, but not rigid squares, that's the limitations on the bilinear algorithm than xnview currently uses.

If you're conflating this with browser fullscreen zoom quality, you're barking up the wrong alley entirely.

Edit:
Okay, I figured out what you mean. In the browser fullscreen, if you have "HQ Enlarge" enabled, you will get an HQ fit to desktop now, but not a continued HQ zoom beyond that. So what you want is for HQ to be extended to other zooms in the (browser) fullscreen mode. (I don't care personally, but at least it's defined now.)

See, this is what happens when you don't report a bug with enough information to reproduce it.

It is related to this previous thread. If there is no more support for adding HQ beyond screen size in Browser's fullscreen, I would suggest to leave it as it is now...