This is the website I'm doing:
The website biggest width is 1080px, it works fine when view in screen which is larger than 1080px (width)...
but when view in screen shorter than 1080px (with a horizontal scrollbar appear) this is what happen, the gray background disappear....
This is the CSS i'm setting up my background and width:
border-top: 1px solid #CECFCE;
1080px fixed width is bad practice as at least 50% or more audience on the web are still using screen resolutions that are less than 1024px wide.
If you're looking at fixed width, you should ideally size your width at 960px max. This is optimal.
You could make your sites based on www.960.gs - the 960 grid system. A tried and tested grid system.
To add to all this, in your case, you have used a BG image on your site that is 1800px wide. I remember already answering one of your previous questions asking you to remove this background.