Issue #2028965 by jessebeach, ekl1773: Fixed Overlay overflows the viewport in...

Issue #2028965 by jessebeach, ekl1773: Fixed Overlay overflows the viewport in Firefox; missing vendor prefixes for box-sizing.

core/modules/overlay/css/overlay-child.css

@@ -19,6 +19,8 @@ html[dir="rtl"] {
 }
 
 #overlay {
+  -webkit-box-sizing: border-box;
+  -moz-box-sizing: border-box;
   box-sizing: border-box;
   display: block;
   margin: 0 auto;