zoom
..are two things that are just not meant to be used together, at least as far as I can tell based on personal experience and several questions on Stack Overflow. You can normally account for the mobile viewport’s or browser window’s size/zoom level by the use of CSS media queries, but those don’t work at all when using pinch-to-zoom on mobile devices.
For example, here are three screenshots of an in-progress layout of this site that uses a fixed div for a sidebar, as viewed on a desktop browser.