I managed to reproduce it on an Android device with Chrome... So try this...
#g-navigation .g-main-nav .g-dropdown {
z-index: 99999;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.
The following users have thanked you: Jessica