Add z-index to skelton content to avoid publish button bug

Adding z-index to body causes Publish button in actions sidebar to appear underneath the layout header causing the “wrong” publish button to be shown. Applying z-index to content avoids this whilst achieving the same outcome in terms of avoiding the bleed of backgrounfd on mobile safari when scrolling the Widgets editor.

packages/interface/src/components/interface-skeleton/style.scss

@@ -63,11 +63,6 @@ html.interface-interface-skeleton__html-container {
 	// See also: https://drafts.csswg.org/css-overscroll/
 	overscroll-behavior-y: none;
 
-	// On Safari iOS on smaller viewports lack of a z-index causes the background
-	// to "bleed" through the header.
-	// See https://github.com/WordPress/gutenberg/issues/32631
-	z-index: z-index(".interface-interface-skeleton__body");
-
 	// Footer overlap prevention
 	.has-footer & {
 		@include break-medium() {
@@ -87,9 +82,13 @@ html.interface-interface-skeleton__html-container {
 	// On Mobile the header is fixed to keep HTML as scrollable.
 	// Beyond the medium breakpoint, we allow the sidebar.
 	// The sidebar should scroll independently, so enable scroll here also.
-
 	overflow: auto;
 
+	// On Safari iOS on smaller viewports lack of a z-index causes the background
+	// to "bleed" through the header.
+	// See https://github.com/WordPress/gutenberg/issues/32631
+	z-index: z-index(".interface-interface-skeleton__body");
+
 }
 
 .interface-interface-skeleton__secondary-sidebar,