Hi,
i'm not quite sure what you mean with dropdown, but you can try the following: In each layout template you will find something like:
<div class="gpxtrackmap">%TRACKMAP%</div>
The %TRACKMAP% placeholder gets replaced with the code for the map, so you can try to set the z-index in the surrounding div tag in the layout template file -OR- include a css rule for div.gpxtrackmap in your css file and set the z-index there.
hth & cheers, Frank