Implement cross overview drag & drop

The gnome-panel allows the user to hover over a tasklist entry
while draging to activate a minimized or obscured window and drop onto it.

Implement a similar behaviour by allowing draging to the activities button or
the hotcorner (and thus opening the overview), which allows the user to
activate any window (even on different workspaces) as a drop target.

https://bugzilla.gnome.org/show_bug.cgi?id=601731

js/ui/xdndHandler.js

@@ -0,0 +1,130 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const Clutter = imports.gi.Clutter;
+const Lang = imports.lang;
+const Shell = imports.gi.Shell;
+const Signals = imports.signals;
+const Mainloop = imports.mainloop;
+const DND = imports.ui.dnd;
+
+function XdndHandler() {
+    this._init();
+}
+
+XdndHandler.prototype = {
+    _init: function() {
+        // Used to display a clone of the cursor window when the
+        // window group is hidden (like it happens in the overview)
+        this._cursorWindowClone = null;
+
+        // Used as a drag actor in case we don't have a cursor window clone
+        this._dummy = new Clutter.Rectangle({ width: 1, height: 1, opacity: 0 });
+        global.stage.add_actor(this._dummy);
+        this._dummy.hide();
+
+        // Mutter delays the creation of the output window as long
+        // as possible to avoid flicker. In case a plugin wants to
+        // access it directly it has to connect to the stage's show
+        // signal. (see comment in compositor.c:meta_compositor_manage_screen)
+        global.stage.connect('show', function () {
+                                        global.init_xdnd();
+                                        return false;
+                                      });
+
+        global.connect('xdnd-enter', Lang.bind(this, this._onEnter));
+        global.connect('xdnd-position-changed', Lang.bind(this, this._onPositionChanged));
+        global.connect('xdnd-leave', Lang.bind(this, this._onLeave));
+
+        this._windowGroupVisibilityHandlerId = 0;
+    },
+
+    // Called when the user cancels the drag (i.e release the button)
+    _onLeave: function() {
+        if (this._windowGroupVisibilityHandlerId != 0) {
+            Mainloop.source_remove(this._windowGroupVisibilityHandlerId);
+            this._windowGroupVisibilityHandlerId = 0;
+        }
+        this.emit('drag-end');
+    },
+
+    _onEnter: function() {
+        this._windowGroupVisibilityHandlerId  =
+                global.window_group.connect('notify::visible',
+                    Lang.bind(this, this._onWindowGroupVisibilityChanged));
+
+        this.emit('drag-begin', global.get_current_time());
+    },
+
+    _onWindowGroupVisibilityChanged: function() {
+        if (!global.window_group.visible) {
+            if (this._cursorWindowClone)
+                return;
+
+            let windows = global.get_window_actors();
+            let cursorWindow = windows[windows.length - 1];
+
+            // FIXME: more reliable way?
+            if (!cursorWindow.is_override_redirect())
+                return;
+
+            let constraint_x = new Clutter.BindConstraint({ coordinate : Clutter.BindCoordinate.X,
+                                                             source: cursorWindow});
+            let constraint_y = new Clutter.BindConstraint({ coordinate : Clutter.BindCoordinate.Y,
+                                                             source: cursorWindow});
+
+            this._cursorWindowClone = new Clutter.Clone({ source: cursorWindow });
+            global.overlay_group.add_actor(this._cursorWindowClone);
+            Shell.util_set_hidden_from_pick(this._cursorWindowClone, true);
+
+            // Make sure that the clone has the same position as the source
+            this._cursorWindowClone.add_constraint(constraint_x);
+            this._cursorWindowClone.add_constraint(constraint_y);
+        } else {
+            if (this._cursorWindowClone)
+            {
+                this._cursorWindowClone.destroy();
+                this._cursorWindowClone = null;
+            }
+        }
+    },
+
+    _onPositionChanged: function(obj, x, y) {
+        let pickedActor = global.stage.get_actor_at_pos(Clutter.PickMode.ALL, x, y);
+
+        // Make sure that the cursor window is on top
+        if (this._cursorWindowClone)
+             this._cursorWindowClone.raise_top();
+
+        let dragEvent = {
+            x: x,
+            y: y,
+            dragActor: this._cursorWindowClone ? this._cursorWindowClone : this._dummy,
+            source: this,
+            targetActor: pickedActor
+        };
+
+        for (let i = 0; i < DND.dragMonitors.length; i++) {
+            let motionFunc = DND.dragMonitors[i].dragMotion;
+            if (motionFunc) {
+                let result = motionFunc(dragEvent);
+                if (result != DND.DragMotionResult.CONTINUE)
+                    return;
+            }
+        }
+
+        while (pickedActor) {
+                if (pickedActor._delegate && pickedActor._delegate.handleDragOver) {
+                    let result = pickedActor._delegate.handleDragOver(this,
+                                                                      dragEvent.dragActor,
+                                                                      x,
+                                                                      y,
+                                                                      global.get_current_time());
+                    if (result != DND.DragMotionResult.CONTINUE)
+                        return;
+                }
+                pickedActor = pickedActor.get_parent();
+        }
+    }
+}
+
+Signals.addSignalMethods(XdndHandler.prototype);