in mobile web application I'd often use modals.
Some modals can expand sizes and thus were made scrollable.
How can I enable higher z-indexed modal to be scrollable and set background div not to scroll?
b/c whenever I scroll with two fingers on the current modal, sometimes background scroll, sometimes foreground scrolls.
I think you can't fix background with background-attachment: fixed;
you have to apply position: fixed on it
and then
You should try iScroll 4
"iScroll finally received a complete rewrite. Now it’s smoother than ever and adds some new important features: pinch/zoom, pull down to refresh, snap to elements and more custom events for a higher level of hackability."
or
Stand-alone parallax scrolling JavaScript library for mobile (Android, iOS, etc.) and desktop in about 12k minified.
Other Helpful link
http://webdesigner-webdeveloper.com/weblog/fullscreen-images-for-the-ipad-ios-and-mobile-safari/
https://github.com/louisremi/background-size-polyfill/issues/27
http://bradfrostweb.com/blog/mobile/fixed-position/
Hope this answer little bit helpful for you. :)