Why does the background disappear when using the device toolbar?

Question

css

Please explain why the background disappears if you change the size of the site along the x-axis in the device toolbar (example in the picture).
Link to the example https://orezraey1337.github.io/
I would be grateful for your help.

Answer 1

Because the background takes the width of the viewport on this screen is 725px, and the entire content of the containers remains 960px, since the width of the container itself is 940 fixed + 20px padding. The view area seems to be moving away, zooming in the opposite direction, not showing a horizontal scroll, because of this, such an effect occurs.