Index: src/wp-content/themes/twentynineteen/sass/navigation/_menu-main-navigation.scss
===================================================================
--- src/wp-content/themes/twentynineteen/sass/navigation/_menu-main-navigation.scss	(revision 47861)
+++ src/wp-content/themes/twentynineteen/sass/navigation/_menu-main-navigation.scss	(working copy)
@@ -205,7 +205,7 @@
 				line-height: $font__line-height-heading;
 				text-shadow: none;
 				padding: calc( .5 * #{$size__spacing-unit} ) calc( 24px + #{$size__spacing-unit} ) calc( .5 * #{$size__spacing-unit} ) $size__spacing-unit;
-				white-space: nowrap;
+				max-width: #{20 * $size__spacing-unit};
 
 				&:hover,
 				&:focus {
Index: src/wp-content/themes/twentynineteen/style.css
===================================================================
--- src/wp-content/themes/twentynineteen/style.css	(revision 47861)
+++ src/wp-content/themes/twentynineteen/style.css	(working copy)
@@ -2959,7 +2959,7 @@
   line-height: 1.2;
   text-shadow: none;
   padding: calc( .5 * 1rem) calc( 24px + 1rem) calc( .5 * 1rem) 1rem;
-  white-space: nowrap;
+  max-width: 20rem;
 }
 
 .main-navigation .sub-menu > li > a:hover, .main-navigation .sub-menu > li > a:focus,