Dropdown list stacking below main content on mobile

Hi Folks,

When on mobile, the size drop down list is stacked below the main content (“Search Results”): https://www.hoppfootwear.com.au/wp-content/uploads/2020/05/woof2.png

I’ve tried CSS:

.chosen-drop {z-index: 99999999;}

but this doesn’t change things.

Help appreciated.

The page I need help with: [log in to see the link]