I think this is happening because (for your div)
max-width:65%;
in the css...if you zoom in enough the button (which has max-width of 100px) is now exceeding the width of your div. Try setting your div to a number of pixels larger than 100px. If you want the button on the right side of the div, try setting the width to a number such as 300px.
max-width:300px;
then try zooming in. Also note IE6 and earlier do not support max-width.