[beta] Safari fixes (#4902)

* Add intitial max-width to sections

* Move background z-index to -1

js/src/ui/Portal/portal.css

@@ -51,7 +51,7 @@ $popoverZ: 3600;
   left: 0;
   right: 0;
   opacity: 0.25;
-  z-index: 0;
+  z-index: -1;
 }
 
 .overlay {

js/src/ui/SectionList/sectionList.css

@@ -44,6 +44,7 @@ $widthExpanded: 42%;
       box-sizing: border-box;
       display: flex;
       flex: 0 1 $widthNormal;
+      max-width: $widthNormal;
       opacity: 0.85;
       padding: 0.25em;