gdk: Implement motion history as motion event data

In the motion compression phase the coalesced events will be saved
as a GdkTimeCoord on the motion event that shall be delivered.

For simplicity (and because history doesn't make much sense otherwise)
event history is only recorded while there are buttons pressed, this
also tidily ensures that those coalesced events would have the same
target widget on the gtk side than the delivered one, because of
implicit grabs.

gdk/gdkevents.c

@@ -337,13 +337,37 @@ _gdk_event_unqueue (GdkDisplay *display)
   return event;
 }
 
+static void
+gdk_event_push_history (GdkEvent       *event,
+                        const GdkEvent *history_event)
+{
+  GdkTimeCoord *hist;
+  GdkDevice *device;
+  gint i, n_axes;
+
+  g_assert (event->any.type == GDK_MOTION_NOTIFY);
+  g_assert (history_event->any.type == GDK_MOTION_NOTIFY);
+
+  hist = g_new0 (GdkTimeCoord, 1);
+
+  device = gdk_event_get_device (history_event);
+  n_axes = gdk_device_get_n_axes (device);
+
+  for (i = 0; i <= MIN (n_axes, GDK_MAX_TIMECOORD_AXES); i++)
+    gdk_event_get_axis (history_event, i, &hist->axes[i]);
+
+  event->motion.history = g_list_prepend (event->motion.history, hist);
+}
+
 void
 _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
 {
   GList *tmp_list;
   GList *pending_motions = NULL;
+  GList *history = NULL;
   GdkWindow *pending_motion_window = NULL;
   GdkDevice *pending_motion_device = NULL;
+  GdkEvent *last_motion = NULL;
 
   /* If the last N events in the event queue are motion notify
    * events for the same window, drop all but the last */
@@ -371,6 +395,9 @@ _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
       if (!event->any.window->event_compression)
         break;
 
+      if (!last_motion)
+        last_motion = event;
+
       pending_motion_window = event->any.window;
       pending_motion_device = event->any.device;
       pending_motions = tmp_list;
@@ -381,6 +408,15 @@ _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
   while (pending_motions && pending_motions->next != NULL)
     {
       GList *next = pending_motions->next;
+
+      history = g_list_prepend (history, pending_motions->data);
+
+      if (last_motion &&
+          (last_motion->motion.state &
+           (GDK_BUTTON1_MASK | GDK_BUTTON2_MASK | GDK_BUTTON3_MASK |
+            GDK_BUTTON4_MASK | GDK_BUTTON5_MASK)))
+        gdk_event_push_history (last_motion, pending_motions->data);
+
       gdk_event_free (pending_motions->data);
       display->queued_events = g_list_delete_link (display->queued_events,
                                                    pending_motions);
@@ -558,6 +594,12 @@ gdk_event_get_pointer_emulated (GdkEvent *event)
   return (event->any.flags & GDK_EVENT_POINTER_EMULATED) != 0;
 }
 
+static GdkTimeCoord *
+copy_time_coord (const GdkTimeCoord *coord)
+{
+  return g_memdup (coord, sizeof (GdkTimeCoord));
+}
+
 /**
  * gdk_event_copy:
  * @event: a #GdkEvent
@@ -642,6 +684,12 @@ gdk_event_copy (const GdkEvent *event)
                                            sizeof (gdouble) * gdk_device_get_n_axes (event->any.device));
       if (event->motion.tool)
         g_object_ref (new_event->motion.tool);
+
+      if (event->motion.history)
+        {
+          new_event->motion.history = g_list_copy_deep (event->motion.history,
+                                                        (GCopyFunc) copy_time_coord, NULL);
+        }
       break;
 
     default:
@@ -718,6 +766,7 @@ gdk_event_finalize (GObject *object)
     case GDK_MOTION_NOTIFY:
       g_clear_object (&event->motion.tool);
       g_free (event->motion.axes);
+      g_list_free_full (event->motion.history, g_free);
       break;
 
     default:
@@ -2623,3 +2672,10 @@ gdk_event_get_axes (GdkEvent  *event,
   return FALSE;
 }
 
+GList *
+gdk_event_get_history (const GdkEvent *event)
+{
+  if (event->any.type != GDK_MOTION_NOTIFY)
+    return NULL;
+  return g_list_reverse (g_list_copy (event->motion.history));
+}

gdk/gdkevents.h

@@ -716,6 +716,7 @@ GDK_AVAILABLE_IN_3_92
 gboolean       gdk_event_get_axes      (GdkEvent  *event,
                                         gdouble  **axes,
                                         guint     *n_axes);
+GList        * gdk_event_get_history   (const GdkEvent  *event);
 
 G_END_DECLS
 

gdk/gdkeventsprivate.h

@@ -121,6 +121,7 @@ struct _GdkEventMotion
   gint16 is_hint;
   GdkDeviceTool *tool;
   gdouble x_root, y_root;
+  GList *history;
 };
 
 /**