XnView Software

The original poster seems to be describing the No multiple view for same file option.

But I can't find the option for Only one window view opened in MP 0.21. In the classic XnView, this option closed the viewer tab when double-clicking or pressing Enter, which prevented multiple view tabs (for different files) from piling up.

Am I missing it?

Right, that option is missing. I too think it's important, but I don't use tabs, so it was not so important for me.

I generally don't use tabs either, but I find the easiest way to close an image & return to the browser is by double clicking. Without this "Only one window view" option, each image stays open in the background, and memory usage quickly goes through the roof.

Of course, I can close images by hitting ESC instead, but it's a bit less convenient.