window-list: Turn workspace thumbs into drop targets

It makes some sense to allow using the workspace indicator for moving
windows between workspaces as well as for workspace switching. This
applies particularly in GNOME classic after we disabled the overview
there, so that there is again a non-shortcut way of moving windows
between workspaces.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/74

extensions/window-list/workspaceIndicator.js

@@ -1,6 +1,8 @@
 /* exported WorkspaceIndicator */
 const { Clutter, Gio, GObject, Meta, St } = imports.gi;
 
+const DND = imports.ui.dnd;
+const Main = imports.ui.main;
 const PanelMenu = imports.ui.panelMenu;
 const PopupMenu = imports.ui.popupMenu;
 
@@ -16,6 +18,31 @@ let WorkspaceThumbnail = GObject.registerClass({
         });
 
         this._index = index;
+        this._delegate = this; // needed for DND
+    }
+
+    acceptDrop(source) {
+        if (!source.realWindow)
+            return false;
+
+        let window = source.realWindow.get_meta_window();
+        this._moveWindow(window);
+        return true;
+    }
+
+    handleDragOver(source) {
+        if (source.realWindow)
+            return DND.DragMotionResult.MOVE_DROP;
+        else
+            return DND.DragMotionResult.CONTINUE;
+    }
+
+
+    _moveWindow(window) {
+        let monitorIndex = Main.layoutManager.findIndexForActor(this);
+        if (monitorIndex != window.get_monitor())
+            window.move_to_monitor(monitorIndex);
+        window.change_workspace_by_index(this._index, false);
     }
 
     on_clicked() {