Sortable: Fix z-index switching from auto to 0

Save `z-index` before saving `opacity`. Setting `opacity` automatically
changes `z-index`.

Fixes #14683
Closes gh-1762

ui/widgets/sortable.js

@@ -279,20 +279,23 @@ return $.widget( "ui.sortable", $.ui.mouse, {
 				$( "<style>*{ cursor: " + o.cursor + " !important; }</style>" ).appendTo( body );
 		}
 
-		if ( o.opacity ) { // opacity option
-			if ( this.helper.css( "opacity" ) ) {
-				this._storedOpacity = this.helper.css( "opacity" );
-			}
-			this.helper.css( "opacity", o.opacity );
-		}
-
+		// We need to make sure to grab the zIndex before setting the
+		// opacity, because setting the opacity to anything lower than 1
+		// causes the zIndex to change from "auto" to 0.
 		if ( o.zIndex ) { // zIndex option
 			if ( this.helper.css( "zIndex" ) ) {
 				this._storedZIndex = this.helper.css( "zIndex" );
 			}
 			this.helper.css( "zIndex", o.zIndex );
 		}
 
+		if ( o.opacity ) { // opacity option
+			if ( this.helper.css( "opacity" ) ) {
+				this._storedOpacity = this.helper.css( "opacity" );
+			}
+			this.helper.css( "opacity", o.opacity );
+		}
+
 		//Prepare scrolling
 		if ( this.scrollParent[ 0 ] !== this.document[ 0 ] &&
 				this.scrollParent[ 0 ].tagName !== "HTML" ) {