I've tried to quickly check existing requests and comments so forgive me if this is already well known or an option I'm unaware of.
Currently with the initial 800x600 white canvas, the image is centered on screen at 100% zoom. It is not possible to free-pan the image coordinates. Ideally it would be possible to force-center the image when it fully fits on-screen (as it currently is) or disable that to free-pan the image to the screen edges. The upper-left image coordinate (0, 0) should be possible to pan to the screen's/paint.net ruler window region's bottom-right corner. The lower-right image coordinate (800, 600) should be possible to pan to the screen's upper-left. This would make the canvas fully off-screen and it would be necessary to re-pan or re-center the canvas according to the offsets displayed on the rulers.
When zoomed to 1000% the current panning coordinate limits present an additional problem when editing the edges of the canvas. The upper-left corner of the canvas is limited to pan to the upper-left corner of the screen and no further. It is impossible to place the canvas corners or edges at the center of the screen when working at a high zoom near the edges of the canvas.
If it were possible to fix these panning coordinate limits by simply replacing the numbers in a limit(x, min, max) call it may be best to start out with only that minimal change. I'm not sure what side-effects may occur if the limits were changed to the far canvas/window/screen edges to allow the canvas to pan fully off the screen. I do appreciate that adding additional options re: canvas free-pan or auto-center may be far more complex.
I see that the "overscroll" option allows the canvas center to be panned to the edge of the window. This isn't the same as allowing for full free-panning all the way off the screen but it does get half-way there. I understand the logic involved and fear of allowing the image to pan fully off screen leaving users who've "lost my canvas" confused. When zoomed above 100% it seems the limit switches to the corner of the canvas at the center of the screen.
The View/Zoom to Window menu has the effect of "re-center/scale canvas". These are of course all sensible trade-offs to find a balanced middle-ground. So I feel like I'm a bit picky to demand any changes from what's already supported. I am left wondering why the option is not a sensible default enabled rather than disabled apart from backward compatibility. It is certainly possible for me to cope with these trade-offs.