[gtk/scroll-compression] gdk: Compress scroll events

[Matthias Clasen <matthiasc src gnome org>]

commit 535df4d6dcfc7397039f5757ace221d51d6d71eb
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Jun 8 18:07:40 2020 -0400

    gdk: Compress scroll events
    
    Only return one accumulated scroll event per frame.
    Compress them by adding up the deltas.
    
    Still missing: a way to capture history, like
    we do for motion events.
    
    Fixes: #2800

 gdk/gdkevents.c    | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 gdk/gdkinternals.h |  1 +
 gdk/gdksurface.c   |  1 +
 3 files changed, 85 insertions(+), 1 deletion(-)
---
diff --git a/gdk/gdkevents.c b/gdk/gdkevents.c
index e1a58006cb..e51ab82721 100644
--- a/gdk/gdkevents.c
+++ b/gdk/gdkevents.c
@@ -644,6 +644,88 @@ gdk_motion_event_push_history (GdkEvent *event,
 
 }
 
+/*
+ * If the last N events in the event queue are smooth scroll events
+ * for the same surface and device, combine them into one.
+ */
+void
+gdk_event_queue_handle_scroll_compression (GdkDisplay *display)
+{
+  GList *l;
+  GdkSurface *surface = NULL;
+  GdkDevice *device = NULL;
+  GdkEvent *last_event = NULL;
+  GList *scrolls = NULL;
+  double delta_x, delta_y;
+  double dx, dy;
+
+  delta_x = delta_y = 0;
+
+  l = g_queue_peek_tail_link (&display->queued_events);
+
+  while (l)
+    {
+      GdkEvent *event = l->data;
+
+      if (event->flags & GDK_EVENT_PENDING)
+        break;
+
+      if (event->event_type != GDK_SCROLL ||
+          gdk_scroll_event_get_direction (event) != GDK_SCROLL_SMOOTH)
+        break;
+
+      if (surface != NULL &&
+          surface != event->surface)
+        break;
+
+      if (device != NULL &&
+          device != event->device)
+        break;
+
+      if (!last_event)
+        last_event = event;
+
+      surface = event->surface;
+      device = event->device;
+      scrolls = l;
+
+      gdk_scroll_event_get_deltas (event, &dx, &dy);
+      delta_x += dx;
+      delta_y += dy;
+
+      l = l->prev;
+    }
+
+  while (scrolls && scrolls->next != NULL)
+    {
+      GList *next = scrolls->next;
+
+      gdk_event_unref (scrolls->data);
+      g_queue_delete_link (&display->queued_events, scrolls);
+      scrolls = next;
+    }
+
+  if (scrolls)
+    {
+      GdkEvent *old_event, *event;
+
+      old_event = scrolls->data;
+
+      event = gdk_scroll_event_new (surface,
+                                    device,
+                                    gdk_event_get_source_device (old_event),
+                                    gdk_event_get_device_tool (old_event),
+                                    gdk_event_get_time (old_event),
+                                    gdk_event_get_modifier_state (old_event),
+                                    delta_x,
+                                    delta_y,
+                                    gdk_scroll_event_is_stop (old_event));
+
+      g_queue_delete_link (&display->queued_events, scrolls);
+      g_queue_push_tail (&display->queued_events, event);
+    }
+}
+
 void
 _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
 {
@@ -710,7 +792,7 @@ _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
     {
       GdkFrameClock *clock = gdk_surface_get_frame_clock (pending_motion_surface);
       if (clock) /* might be NULL if surface was destroyed */
-       gdk_frame_clock_request_phase (clock, GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS);
+        gdk_frame_clock_request_phase (clock, GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS);
     }
 }
 
diff --git a/gdk/gdkinternals.h b/gdk/gdkinternals.h
index e30872edc6..b26aac73eb 100644
--- a/gdk/gdkinternals.h
+++ b/gdk/gdkinternals.h
@@ -123,6 +123,7 @@ GList* _gdk_event_queue_append       (GdkDisplay *display,
                                       GdkEvent   *event);
 
 void    _gdk_event_queue_handle_motion_compression (GdkDisplay *display);
+void    gdk_event_queue_handle_scroll_compression  (GdkDisplay *display);
 void    _gdk_event_queue_flush                     (GdkDisplay       *display);
 
 gboolean        _gdk_cairo_surface_extents       (cairo_surface_t *surface,
diff --git a/gdk/gdksurface.c b/gdk/gdksurface.c
index 1c5aeb180a..9e5946794d 100644
--- a/gdk/gdksurface.c
+++ b/gdk/gdksurface.c
@@ -2366,6 +2366,7 @@ _gdk_windowing_got_event (GdkDisplay *display,
    * candidate it queues up flushing the event queue.
    */
   _gdk_event_queue_handle_motion_compression (display);
+  gdk_event_queue_handle_scroll_compression (display);
 }
 
 /**