Customize: Prevent preview iframe from showing until fully loaded.

Fixes issue where the Customizer preview window can appear to momentarily bounce to the top when being refreshed. Regression from [36532].

See #31195.
Fixes #36276.

Built from https://develop.svn.wordpress.org/trunk@37060


git-svn-id: http://core.svn.wordpress.org/trunk@37027 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls-rtl.css

@@ -613,6 +613,9 @@ p.customize-section-description {
 	height: 100%;
 	position: absolute;
 }
+#customize-preview iframe + iframe {
+	visibility: hidden;
+}
 
 .wp-full-overlay-sidebar {
 	background: #eee;

wp-admin/css/customize-controls.css

@@ -613,6 +613,9 @@ p.customize-section-description {
 	height: 100%;
 	position: absolute;
 }
+#customize-preview iframe + iframe {
+	visibility: hidden;
+}
 
 .wp-full-overlay-sidebar {
 	background: #eee;