workspaceThumbnail: Sync indicator with WorkspacesDisplay

Now that both ThumbnailsBox and WorkspacesDisplay use single adjustments for
controlling indicator and scrolling, create the adjustment in OverviewControls
and pass it to both objects, effectively syncing indicator to scrolling.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/821

js/ui/overviewControls.js

@@ -436,11 +436,28 @@ class ControlsManager extends St.Widget {
         this._dashSpacer = new DashSpacer();
         this._dashSpacer.setDashActor(this._dashSlider);
 
-        this._thumbnailsBox = new WorkspaceThumbnail.ThumbnailsBox();
+        let workspaceManager = global.workspace_manager;
+        let activeWorkspaceIndex = workspaceManager.get_active_workspace_index();
+
+        this._workspaceAdjustment = new St.Adjustment({
+            value: activeWorkspaceIndex,
+            lower: 0,
+            page_increment: 1,
+            page_size: 1,
+            step_increment: 0,
+            upper: workspaceManager.n_workspaces,
+        });
+
+        this._nWorkspacesNotifyId =
+            workspaceManager.connect('notify::n-workspaces',
+                this._updateAdjustment.bind(this));
+
+        this._thumbnailsBox =
+            new WorkspaceThumbnail.ThumbnailsBox(this._workspaceAdjustment);
         this._thumbnailsSlider = new ThumbnailsSlider(this._thumbnailsBox);
 
         this.viewSelector = new ViewSelector.ViewSelector(searchEntry,
-                                                          this.dash.showAppsButton);
+            this._workspaceAdjustment, this.dash.showAppsButton);
         this.viewSelector.connect('page-changed', this._setVisibility.bind(this));
         this.viewSelector.connect('page-empty', this._onPageEmpty.bind(this));
 
@@ -457,6 +474,24 @@ class ControlsManager extends St.Widget {
         layout.connect('allocation-changed', this._updateWorkspacesGeometry.bind(this));
 
         Main.overview.connect('showing', this._updateSpacerVisibility.bind(this));
+
+        this.connect('destroy', this._onDestroy.bind(this));
+    }
+
+    _onDestroy() {
+        global.workspace_manager.disconnect(this._nWorkspacesNotifyId);
+    }
+
+    _updateAdjustment() {
+        let workspaceManager = global.workspace_manager;
+        let newNumWorkspaces = workspaceManager.n_workspaces;
+        let activeIndex = workspaceManager.get_active_workspace_index();
+
+        this._workspaceAdjustment.upper = newNumWorkspaces;
+
+        // A workspace might have been inserted or removed before the active
+        // one, causing the adjustment to go out of sync, so update the value
+        this._workspaceAdjustment.value = activeIndex;
     }
 
     _updateWorkspacesGeometry() {