scrolledwindow: Trigger builtin kinetic deceleration on libinput devices

The libinput driver will send a 0/0 scroll event on touchpads and other
devices where it knows scrolling stopped for sure. Use these events to
trigger kinetic scrolling from there.

The mechanism is similar to GtkGestureSwipe, we keep a backlog of the
latest dx/dy till a previous point in time, and calculate the final
velocities from there, with the difference we're dealing with scroll
units, and not pixel distances.

https://bugzilla.gnome.org/show_bug.cgi?id=749770

gtk/gtkscrolledwindow.c

@@ -149,6 +149,7 @@
 #define MAX_OVERSHOOT_DISTANCE 100
 #define DECELERATION_FRICTION 4
 #define OVERSHOOT_FRICTION 20
+#define SCROLL_CAPTURE_THRESHOLD_MS 150
 
 /* Animated scrolling */
 #define ANIMATION_DURATION 200
@@ -180,6 +181,13 @@ typedef struct
   guint      over_timeout_id;
 } Indicator;
 
+typedef struct
+{
+  gdouble dx;
+  gdouble dy;
+  guint32 evtime;
+} ScrollHistoryElem;
+
 struct _GtkScrolledWindowPrivate
 {
   GtkWidget     *hscrollbar;
@@ -208,6 +216,9 @@ struct _GtkScrolledWindowPrivate
   GtkGesture *long_press_gesture;
   GtkGesture *swipe_gesture;
 
+  GArray *scroll_history;
+  GdkDevice *scroll_device;
+
   /* These two gestures are mutually exclusive */
   GtkGesture *drag_gesture;
   GtkGesture *pan_gesture;
@@ -869,16 +880,16 @@ scrolled_window_drag_end_cb (GtkScrolledWindow *scrolled_window,
 }
 
 static void
-scrolled_window_swipe_cb (GtkScrolledWindow *scrolled_window,
+gtk_scrolled_window_decelerate (GtkScrolledWindow *scrolled_window,
-                          gdouble            x_velocity,
+                                gdouble            x_velocity,
-                          gdouble            y_velocity)
+                                gdouble            y_velocity)
 {
   GtkScrolledWindowPrivate *priv = scrolled_window->priv;
   gboolean overshoot;
 
   overshoot = _gtk_scrolled_window_get_overshoot (scrolled_window, NULL, NULL);
-  priv->x_velocity = -x_velocity;
+  priv->x_velocity = x_velocity;
-  priv->y_velocity = -y_velocity;
+  priv->y_velocity = y_velocity;
 
   /* Zero out vector components for which we don't scroll */
   if (!may_hscroll (scrolled_window))
@@ -893,6 +904,14 @@ scrolled_window_swipe_cb (GtkScrolledWindow *scrolled_window,
     }
 }
 
+static void
+scrolled_window_swipe_cb (GtkScrolledWindow *scrolled_window,
+                          gdouble            x_velocity,
+                          gdouble            y_velocity)
+{
+  gtk_scrolled_window_decelerate (scrolled_window, -x_velocity, -y_velocity);
+}
+
 static void
 scrolled_window_long_press_cb (GtkScrolledWindow *scrolled_window,
                                gdouble            x,
@@ -1114,6 +1133,87 @@ get_scroll_unit (GtkScrolledWindow *sw,
   return scroll_unit;
 }
 
+static void
+scroll_history_push (GtkScrolledWindow *sw,
+                     GdkEventScroll    *event)
+{
+  GtkScrolledWindowPrivate *priv = sw->priv;
+  ScrollHistoryElem new_item;
+  guint i;
+
+  if (event->direction != GDK_SCROLL_SMOOTH)
+    return;
+
+  for (i = 0; i < priv->scroll_history->len; i++)
+    {
+      ScrollHistoryElem *elem;
+
+      elem = &g_array_index (priv->scroll_history, ScrollHistoryElem, i);
+
+      if (elem->evtime >= event->time - SCROLL_CAPTURE_THRESHOLD_MS)
+        break;
+    }
+
+  if (i > 0)
+    g_array_remove_range (priv->scroll_history, 0, i);
+
+  new_item.dx = event->delta_x;
+  new_item.dy = event->delta_y;
+  new_item.evtime = event->time;
+  g_array_append_val (priv->scroll_history, new_item);
+}
+
+static void
+scroll_history_reset (GtkScrolledWindow *sw)
+{
+  GtkScrolledWindowPrivate *priv = sw->priv;
+
+  if (priv->scroll_history->len == 0)
+    return;
+
+  g_array_remove_range (priv->scroll_history, 0,
+                        priv->scroll_history->len);
+}
+
+static gboolean
+scroll_history_finish (GtkScrolledWindow *sw,
+                       gdouble           *velocity_x,
+                       gdouble           *velocity_y)
+{
+  GtkScrolledWindowPrivate *priv = sw->priv;
+  gdouble accum_dx = 0, accum_dy = 0;
+  guint32 first = 0, last = 0;
+  gdouble xunit, yunit;
+  guint i;
+
+  if (priv->scroll_history->len == 0)
+    return FALSE;
+
+  for (i = 0; i < priv->scroll_history->len; i++)
+    {
+      ScrollHistoryElem *elem;
+
+      elem = &g_array_index (priv->scroll_history, ScrollHistoryElem, i);
+      accum_dx += elem->dx;
+      accum_dy += elem->dy;
+      last = elem->evtime;
+
+      if (i == 0)
+        first = elem->evtime;
+    }
+
+  if (last == first)
+    return FALSE;
+
+  xunit = get_scroll_unit (sw, GTK_ORIENTATION_HORIZONTAL);
+  yunit = get_scroll_unit (sw, GTK_ORIENTATION_VERTICAL);
+  *velocity_x = (accum_dx * 1000 * xunit) / (last - first);
+  *velocity_y = (accum_dy * 1000 * yunit) / (last - first);
+  scroll_history_reset (sw);
+
+  return TRUE;
+}
+
 static gboolean
 captured_event_cb (GtkWidget *widget,
                    GdkEvent  *event)
@@ -1127,20 +1227,31 @@ captured_event_cb (GtkWidget *widget,
 
   sw = GTK_SCROLLED_WINDOW (widget);
   priv = sw->priv;
+  source_device = gdk_event_get_source_device (event);
 
   if (event->type == GDK_SCROLL)
     {
-      gdouble dx, dy;
+      gdouble dx, dy, vel_x, vel_y;
 
       /* The libinput driver may generate a final event with dx=dy=0
-       * after scrolling finished, this is usually an indication that
+       * after scrolling finished, start kinetic scrolling when this
-       * the deceleration animation just started, so we definitely
+       * happens.
-       * shouldn't cancel it.
        */
-      if (event->scroll.direction != GDK_SCROLL_SMOOTH ||
+      gtk_scrolled_window_cancel_deceleration (sw);
-          (gdk_event_get_scroll_deltas (event, &dx, &dy) &&
+      gdk_event_get_scroll_deltas (event, &dx, &dy);
-           (dx != 0 || dy != 0)))
+
-        gtk_scrolled_window_cancel_deceleration (sw);
+      if (priv->scroll_device != source_device)
+        {
+          priv->scroll_device = source_device;
+          scroll_history_reset (sw);
+        }
+
+      scroll_history_push (sw, &event->scroll);
+
+      if (event->scroll.direction == GDK_SCROLL_SMOOTH &&
+          dx == 0 && dy == 0 &&
+          scroll_history_finish (sw, &vel_x, &vel_y))
+        gtk_scrolled_window_decelerate (sw, vel_x, vel_y);
 
       return GDK_EVENT_PROPAGATE;
     }
@@ -1152,7 +1263,6 @@ captured_event_cb (GtkWidget *widget,
       event->type != GDK_LEAVE_NOTIFY)
     return GDK_EVENT_PROPAGATE;
 
-  source_device = gdk_event_get_source_device (event);
   input_source = gdk_device_get_source (source_device);
 
   if (input_source == GDK_SOURCE_KEYBOARD ||
@@ -1265,6 +1375,8 @@ gtk_scrolled_window_init (GtkScrolledWindow *scrolled_window)
                             G_CALLBACK (scrolled_window_long_press_cancelled_cb),
                             scrolled_window);
 
+  priv->scroll_history = g_array_new (FALSE, FALSE, sizeof (ScrollHistoryElem));
+
   gtk_scrolled_window_set_kinetic_scrolling (scrolled_window, TRUE);
   gtk_scrolled_window_set_capture_button_press (scrolled_window, TRUE);
 
@@ -1877,6 +1989,7 @@ gtk_scrolled_window_destroy (GtkWidget *widget)
   g_clear_object (&priv->drag_gesture);
   g_clear_object (&priv->swipe_gesture);
   g_clear_object (&priv->long_press_gesture);
+  g_clear_pointer (&priv->scroll_history, (GDestroyNotify) g_array_unref);
 
   GTK_WIDGET_CLASS (gtk_scrolled_window_parent_class)->destroy (widget);
 }
@@ -2966,7 +3079,10 @@ start_scroll_deceleration_cb (gpointer user_data)
   GtkScrolledWindowPrivate *priv = scrolled_window->priv;
 
   priv->scroll_events_overshoot_id = 0;
-  gtk_scrolled_window_start_deceleration (scrolled_window);
+
+  if (!priv->deceleration_id)
+    gtk_scrolled_window_start_deceleration (scrolled_window);
+
   return FALSE;
 }