workspacesView: Center extra workspace

At the moment views on non-primary monitors take up the entire work area,
so simply allocating the available size works. However we'll soon shrink
the views a bit to match the visuals on the primary monitor. As workspaces
keep the ratio, reducing their height will also reduce the width; override
the default allocate() to keep the extra workspace horizontally centered.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1735>

js/ui/workspacesView.js

@@ -560,6 +560,18 @@ class ExtraWorkspaceView extends WorkspacesViewBase {
         this._workspace.stateAdjustment.value = progress;
     }
 
+    vfunc_allocate(box) {
+        this.set_allocation(box);
+
+        const [width, height] = box.get_size();
+        const [, childWidth] = this._workspace.get_preferred_width(height);
+
+        const childBox = new Clutter.ActorBox();
+        childBox.set_origin(Math.round((width - childWidth) / 2), 0);
+        childBox.set_size(childWidth, height);
+        this._workspace.allocate(childBox);
+    }
+
     getActiveWorkspace() {
         return this._workspace;
     }