Question about XGIMI Horizon S Max (CH)

Exactly. The halo is visible as soon as the projector is not exactly centred, whether horizontally or vertically. I'm very disappointed about the very visible halo. Plus, you always have digital degradation if you project off-centre... the correction is always digital and not mechanical!

Just to add to benmuenst's answer. I agree that the halo is somewhat annoying but you will have that with any projector using digital keystone correction and digital zoom, provided it is bright enough. Having said that, I do use digital keystoning as I have the projector placed to the right of the projection area and hence do get a trapezoid halo around the actual image. However, I don't feel like it impacts much on the viewing experience. In other words, you get used to it. ;-) Also, you know that you can simply move the projector to a dead centre position and not using digital zoom, should you want maximum quality, which is easily done thanks to its portability. This is clearly the best way around the issue.
The second best way around it is to lower the brightness of the image to the point at which you don't see the frame around the image anymore. In fact, I don't see the halo when the room is reasonably well lit (natural light or lamps). The effect will be most pronounced in a completely darkened room at maximum brightness.
In my experience, the loss of resolution / image quality is barely noticeable. I am projecting straight onto the wall and for my application 4K is probably an overkill anyway.
Overall, the advantages of the automatic digital autofocus, keystoning and further image correction options outweigh by far the disadvantages of the halo created in the process. I hope that helps.