Bug 1428839 - Part 8 - Fix the sliding transition when views are reordered. r=Gijs

Views moved to a different panel and then moved back could be placed after the subviews they give access to, if the other subviews were not moved. The transition would be incorrect when these subviews are opened later.

MozReview-Commit-ID: 6JJa0p0McxL

--HG--
extra : rebase_source : 7e28c0638c46b26c45b1073d8a76173fa75fd5fa

browser/components/customizableui/PanelMultiView.jsm

@@ -868,8 +868,10 @@ var PanelMultiView = class extends this.AssociatedToNode {
                    height: nextPanelView.knownHeight };
       nextPanelView.visible = true;
     } else if (viewNode.customRectGetter) {
-      // Can't use Object.assign directly with a DOM Rect object because its properties
-      // aren't enumerable.
+      // We use a customRectGetter for WebExtensions panels, because they need
+      // to query the size from an embedded browser. The presence of this
+      // getter also provides an indication that the view node shouldn't be
+      // moved around, otherwise the state of the browser would get disrupted.
       let width = prevPanelView.knownWidth;
       let height = prevPanelView.knownHeight;
       viewRect = Object.assign({height, width}, viewNode.customRectGetter());
@@ -880,7 +882,6 @@ var PanelMultiView = class extends this.AssociatedToNode {
       nextPanelView.visible = true;
       nextPanelView.descriptionHeightWorkaround();
     } else {
-      let oldSibling = viewNode.nextSibling || null;
       this._offscreenViewStack.style.minHeight = olderView.knownHeight + "px";
       this._offscreenViewStack.appendChild(viewNode);
       nextPanelView.visible = true;
@@ -897,11 +898,9 @@ var PanelMultiView = class extends this.AssociatedToNode {
         return;
       }
 
-      try {
-        this._viewStack.insertBefore(viewNode, oldSibling);
-      } catch (ex) {
-        this._viewStack.appendChild(viewNode);
-      }
+      // Place back the view after all the other views that are already open in
+      // order for the transition to work as expected.
+      this._viewStack.appendChild(viewNode);
 
       this._offscreenViewStack.style.removeProperty("min-height");
     }