fix z-index problem

src/Umbraco.Web.UI.Client/src/less/components/editor.less

@@ -66,7 +66,7 @@
 	background: @white;
 	position: absolute;
 	padding: 0 20px;
-	z-index: @zindexFixedNavbar;
+	z-index: @zIndexEditor;
 	border-bottom: 1px solid @gray-9;
 	width: 100%;
 	box-sizing: border-box;

src/Umbraco.Web.UI.Client/src/less/components/editor/umb-editor.less

@@ -13,7 +13,7 @@
     bottom: 0;
     left: 0;
     background: @gray-10;
-    z-index: 100;
+    z-index: @zIndexEditor;
 }
 
 .umb-editor--animating {
@@ -43,5 +43,5 @@
     right: 0;
     left: 0;
     background: rgba(0,0,0,0.2);
-    z-index: 100;
+    z-index: @zIndexEditor;
 }

src/Umbraco.Web.UI.Client/src/less/variables.less

@@ -319,6 +319,7 @@
 // -------------------------
 // Used for a bird's eye view of components dependent on the z-axis
 // Try to avoid customizing these :)
+@zIndexEditor:             100;
 @zIndexTree:               100;
 @zindexDropdown:          1000;
 @zindexPopover:           1010;