Allow dropping windows directly to dock

src/DBus.vala

@@ -18,7 +18,7 @@ public class Gala.DBus {
             SESSION, "io.elementary.gala", NONE,
             (connection) => {
                 try {
-                    connection.register_object ("/io/elementary/gala", WindowDragProvider.get_instance ());
+                    connection.register_object ("/io/elementary/gala", WindowDragProvider.get_instance (wm.get_display ()));
                 } catch (Error e) {
                     warning (e.message);
                 }

src/DesktopIntegration.vala

@@ -168,6 +168,8 @@ public class Gala.DesktopIntegration : GLib.Object {
     public void move_window_to_workspace (uint64 uid, int index) throws DBusError, IOError {
         var window = find_window_by_uid (uid);
         window.change_workspace_by_index (index, false);
+
+        WindowDragProvider.get_instance (wm.get_display ()).handle_move (uid);
     }
 
     public void activate_workspace (int index) throws GLib.DBusError, GLib.IOError {

src/Widgets/MultitaskingView/WindowClone.vala

@@ -506,14 +506,14 @@ public class Gala.WindowClone : ActorTarget, RootTarget {
         }
 
         if (hovered) {
-            WindowDragProvider.get_instance ().notify_enter (window.get_id ());
+            WindowDragProvider.get_instance (wm.get_display ()).notify_enter (window.get_id ());
         } else {
-            WindowDragProvider.get_instance ().notify_leave ();
+            WindowDragProvider.get_instance (wm.get_display ()).notify_leave ();
         }
     }
 
     private void destination_motion (Clutter.Actor destination, float x, float y) {
-        WindowDragProvider.get_instance ().notify_motion (x, y);
+        WindowDragProvider.get_instance (wm.get_display ()).notify_motion (x, y);
     }
 
     /**
@@ -549,7 +549,7 @@ public class Gala.WindowClone : ActorTarget, RootTarget {
                 did_move = true;
             }
         } else if (destination is Meta.WindowActor) {
-            WindowDragProvider.get_instance ().notify_dropped ();
+            WindowDragProvider.get_instance (display).notify_dropped ();
         }
 
         if (did_move) {

src/WindowDragProvider.vala

@@ -8,15 +8,93 @@
 [DBus (name = "io.elementary.desktop.wm.WindowDragProvider")]
 public class Gala.WindowDragProvider : Object {
     private static GLib.Once<WindowDragProvider> instance;
-    public static WindowDragProvider get_instance () {
-        return instance.once (() => new WindowDragProvider ());
+    public static WindowDragProvider get_instance (Meta.Display display) {
+        return instance.once (() => new WindowDragProvider (display));
     }
 
     public signal void enter (uint64 window_id);
     public signal void motion (int x, int y);
     public signal void leave ();
     public signal void dropped ();
 
+    [DBus (visible = false)]
+    public Meta.Display display { private get; construct; }
+
+    private ulong position_invalidated_id = 0;
+    private Meta.Window? previous_dock_window = null;
+    private Meta.Window? window_waiting_to_move = null;
+
+    public WindowDragProvider (Meta.Display display) {
+        Object (display: display);
+    }
+
+    construct {
+        display.grab_op_begin.connect ((grabbed_window, grab_op) => {
+            if (grab_op != MOVING) {
+                return;
+            }
+
+#if HAS_MUTTER48
+            unowned var cursor_tracker = display.get_compositor ().get_backend ().get_cursor_tracker ();
+#else
+            unowned var cursor_tracker = display.get_cursor_tracker ();
+#endif
+            position_invalidated_id = cursor_tracker.position_invalidated.connect ((cursor_tracker) => {
+                Graphene.Point pointer;
+                cursor_tracker.get_pointer (out pointer, null);
+
+                foreach (unowned var window in display.list_all_windows ()) {
+                    if (window.window_type != DOCK) {
+                        continue;
+                    }
+                        var buffer_rect = window.get_buffer_rect ();
+#if HAS_MUTTER48
+                    if (buffer_rect.contains_pointf (pointer.x, pointer.y)) {
+#else
+                    if (buffer_rect.contains_rect ({ (int) pointer.x, (int) pointer.y, 0, 0})) {
+#endif
+                        if (previous_dock_window != window) {
+                            notify_enter (grabbed_window.get_id ());
+                            previous_dock_window = window;
+                        } else {
+                            notify_motion ((int) pointer.x - buffer_rect.x, (int) pointer.y - buffer_rect.y);
+                        }
+
+                        return;
+                    }
+                }
+
+                if (previous_dock_window != null) {
+                    notify_leave ();
+                    previous_dock_window = null;
+                }
+            });
+        });
+
+        display.grab_op_end.connect ((grabbed_window, grab_op) => {
+            if (grab_op != MOVING) {
+                return;
+            }
+
+            if (position_invalidated_id > 0) {
+#if HAS_MUTTER48
+                unowned var cursor_tracker = display.get_compositor ().get_backend ().get_cursor_tracker ();
+#else
+                unowned var cursor_tracker = display.get_cursor_tracker ();
+#endif
+                cursor_tracker.disconnect (position_invalidated_id);
+                position_invalidated_id = 0;
+
+                if (previous_dock_window != null) {
+                    notify_dropped ();
+                    notify_leave ();
+                    previous_dock_window = null;
+                    window_waiting_to_move = grabbed_window;
+                }
+            }
+        });
+    }
+
     internal void notify_enter (uint64 window_id) {
         enter (window_id);
     }
@@ -32,4 +110,29 @@ public class Gala.WindowDragProvider : Object {
     internal void notify_dropped () {
         dropped ();
     }
+
+    /**
+     * Handles centering the window on the workspace in case it was dragged directly to the dock.
+     * If we don't do that, the dragged window will sit awkwardly at the bottom of the monitor.
+     */
+    internal void handle_move (uint64 uid) {
+        if (uid != window_waiting_to_move.get_id ()) {
+            warning ("WindowDragProvider: Windows id don't match");
+            window_waiting_to_move = null;
+            return;
+        }
+
+        var frame = window_waiting_to_move.get_frame_rect ();
+        var monitor_geometry = display.get_monitor_geometry (window_waiting_to_move.get_monitor ());
+
+        window_waiting_to_move.move_resize_frame (
+            true,
+            monitor_geometry.x + (monitor_geometry.width - frame.width) / 2,
+            monitor_geometry.y + (monitor_geometry.height - frame.height) / 2,
+            frame.width,
+            frame.height
+        );
+
+        window_waiting_to_move = null;
+    }
 }