fix(navbar): ensure .navbar-collapse behaves as intended (twbs#33022)

Author: ffoodd

scss/_navbar.scss

@@ -109,6 +109,7 @@
 // the default flexbox row orientation. Requires the use of `flex-wrap: wrap`
 // on the `.navbar` parent.
 .navbar-collapse {
+  flex-basis: 100%;
   flex-grow: 1;
   // For always expanded or extra full navbars, ensure content aligns itself
   // properly vertically. Can be easily overridden with flex utilities.
@@ -185,6 +186,7 @@
 
         .navbar-collapse {
           display: flex !important; // stylelint-disable-line declaration-no-important
+          flex-basis: auto;
         }
 
         .navbar-toggler {