Salient 8.5.4 is Now Compatible with Woocommerce 3.3.1 . Kindly update the Theme if you have updated Woocommerce . 

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“Sticky Mobile Menu”

add this into your custom css box located in the Salient options panel:

@media only screen and (min-width : 1px) and (max-width : 1000px) {
   body #header-space {
      height: 56px!important;
      display: block!important;
   }
   body #header-outer {
        position: fixed !important;
   }
   #mobile-menu {
     position: fixed!important;
     top: 52px!important;
     width: 100%!important;
   }
}

Note: Might be incompatible with some smartphones .