A jquery div has problem on mobile device

I use a Jquery DIV that (when you scroll down the page) move on top and stop when it reaches the top of the page. This DIV gave some problems on mobile devices or on too small screens so I want use these DIV for all browser and use another div (or another div style) if user uses a screen too small or a mobile device. The work should be done using conditional comments in the style sheet or through an alternative technique. See atthached file as exsample. considered that if you use a conditional comment in the spreadsheet style this comment should apply only to this DIV and not for the entire style sheet.