workspaceAnimation: Sync progress with main adjustment

Create a derived workspaces adjustment using the new API, and
bind it to the 'progress' property. This is only done by the
MonitorGroup representing the primary monitor.

The progress value that WorkspaceAnimation operates on
represents the percentage within the workspaceIndices array
we are, so make sure to transform the percentage to the
correct workspace index.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2881>

js/ui/workspaceAnimation.js

@@ -9,6 +9,7 @@ import St from 'gi://St';
 import * as Background from './background.js';
 import * as Layout from './layout.js';
 import * as SwipeTracker from './swipeTracker.js';
+import * as Util from '../misc/util.js';
 
 import * as Main from './main.js';
 
@@ -187,6 +188,25 @@ const MonitorGroup = GObject.registerClass({
         }
 
         this.progress = this.getWorkspaceProgress(activeWorkspace);
+
+        if (monitor.index === Main.layoutManager.primaryIndex) {
+            this._workspacesAdjustment = Main.createWorkspacesAdjustment(this);
+            this.bind_property_full('progress',
+                this._workspacesAdjustment, 'value',
+                GObject.BindingFlags.SYNC_CREATE,
+                (bind, source) => {
+                    const indices = [
+                        workspaceIndices[Math.floor(source)],
+                        workspaceIndices[Math.ceil(source)],
+                    ];
+                    return [true, Util.lerp(...indices, source % 1.0)];
+                },
+                null);
+
+            this.connect('destroy', () => {
+                delete this._workspacesAdjustment;
+            });
+        }
     }
 
     get baseDistance() {