MDL-85287 User tours: Avoid scrolling while backdrop is displayed

If the page scrolls while the backdrop is displayed, it looks
incorrect because the backdrop does not scroll with page content.
To avoid this, don't show the backdrop until scrolling has finished.

public/admin/tool/usertours/amd/src/tour.js

@@ -808,9 +808,6 @@ const Tour = class {
 
             targetNode.data('flexitour', 'target');
 
-            // Add the backdrop.
-            this.positionBackdrop(stepConfig);
-
             $(document.body).append(currentStepNode);
             this.currentStepNode = currentStepNode;
 
@@ -826,6 +823,7 @@ const Tour = class {
                 .animate({
                     scrollTop: this.calculateScrollTop(stepConfig),
                 }).promise().then(function() {
+                        this.positionBackdrop(stepConfig);
                         this.positionStep(stepConfig);
                         this.revealStep(stepConfig);
                         pendingPromise.resolve();