I made a shadow under the box with a Gaussian blur. When you look on your mobile it looks good. But if you look from the computer monitor at a certain angle, an ugly frame like a halo appears around the shadow. What is the matter and how to solve such a problem?
This is caused by the lighting technology of the monitor.
There's nothing you can do other than change the screen you are viewing things on or stop looking at it from an angle on that monitor. There's no adjustment you can make to the image itself which will alter the technology of the monitor.
You are essentially asking how to fix your automobile because potholes in the road are making the ride bumpy. You can't correct the delivery system by adjusting the object being delivered. That's just how physics works.
Unless you can give us details on the file, format, hardware, time of day, pretty much anything and everything we can't help you out. This is highly specific situation where it's really a hardware thing and how light and modern displays work and how your SPECIFIC set-up is.
To expand on Scott's answer a little more technically. What you are seeing "COULD" be based on the Pixel Density (ppi, px/cm), Pixel Size (mm), or the Angular pixel density (px/° at typ. distance) or all 3.
Then the viewing distance is a factor too. All displays have a Typical Viewing Distance (typ.) which affects how images are viewed by the human eye.
This all can be used to calculate the PPD or Pixels-Per-Degree.
d is the distance to the screen, r is the resolution of the screen in pixels per unit length, and 1° is the aperture of a cone having the apex on focus, height d, and the base in the eye lens
All this equation means is that the aperture varies widely from person to person.
All this means is that it's 100% arbitrary and can then also be further complicated by design and even the method of viewing. Is this on Chrome? Firefox? Safari? Each browser has its own method of rendering images.
It could be OLED degradation for all I know.
Literally nothing I've said should be taken as advice or help, it's purely wild speculation based loosely off of the EXTREMELY limited information given.