[gtk/wip/carlosg/for-master: 6/8] gtk/scrolledwindow: Handle full scroll sequences only

[Carlos Garnacho <carlosg src gnome org>]

commit e9fe270e94d6dbae4655f8a31910f9fb7288d457
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Dec 12 00:40:12 2020 +0100

    gtk/scrolledwindow: Handle full scroll sequences only
    
    In order to do this, leverage smooth scroll handling into the capture
    phase scroll controller, controlled by ::scroll-begin in the propagation
    phase controller.
    
    There's 2 cases here:
    - A child widget handles scroll. The scrolled window does not get
      ::scroll-begin, the child widget handles the full scroll sequence.
    - No child handles scroll, the scrolled window gets ::scroll-begin,
      and transfers control of scrolling to the capture phase controller.
      As scrolling is performed, the pointer may fall into scrollable children,
      but the scrolled window will be capturing the scroll events, so these
      won't be seen by the child widgets.
    
    Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/593

 gtk/gtkscrolledwindow.c | 44 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 8 deletions(-)
---
diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index ffefb0f7d9..d27d05c432 100644
--- a/gtk/gtkscrolledwindow.c
+++ b/gtk/gtkscrolledwindow.c
@@ -408,6 +408,11 @@ static void     indicator_set_over   (Indicator *indicator,
 static void     install_scroll_cursor (GtkScrolledWindow *scrolled_window);
 static void     uninstall_scroll_cursor (GtkScrolledWindow *scrolled_window);
 
+static void scrolled_window_scroll (GtkScrolledWindow        *scrolled_window,
+                                    double                    delta_x,
+                                    double                    delta_y,
+                                    GtkEventControllerScroll *scroll);
+
 static guint signals[LAST_SIGNAL] = {0};
 static GParamSpec *properties[NUM_PROPERTIES];
 
@@ -1220,8 +1225,17 @@ captured_scroll_cb (GtkEventControllerScroll *scroll,
                     double                    delta_y,
                     GtkScrolledWindow        *scrolled_window)
 {
+  GtkScrolledWindowPrivate *priv =
+    gtk_scrolled_window_get_instance_private (scrolled_window);
+
   gtk_scrolled_window_cancel_deceleration (scrolled_window);
 
+  if (priv->smooth_scroll)
+    {
+      scrolled_window_scroll (scrolled_window, delta_x, delta_y, scroll);
+      return GDK_EVENT_STOP;
+    }
+
   return GDK_EVENT_PROPAGATE;
 }
 
@@ -1305,13 +1319,14 @@ scroll_controller_scroll_begin (GtkEventControllerScroll *scroll,
   priv->smooth_scroll = TRUE;
 }
 
-static gboolean
-scroll_controller_scroll (GtkEventControllerScroll *scroll,
-                          double                    delta_x,
-                          double                    delta_y,
-                          GtkScrolledWindow        *scrolled_window)
+static void
+scrolled_window_scroll (GtkScrolledWindow        *scrolled_window,
+                        double                    delta_x,
+                        double                    delta_y,
+                        GtkEventControllerScroll *scroll)
 {
-  GtkScrolledWindowPrivate *priv = gtk_scrolled_window_get_instance_private (scrolled_window);
+  GtkScrolledWindowPrivate *priv =
+    gtk_scrolled_window_get_instance_private (scrolled_window);
   gboolean shifted;
   GdkModifierType state;
 
@@ -1373,6 +1388,19 @@ scroll_controller_scroll (GtkEventControllerScroll *scroll,
       g_source_set_name_by_id (priv->scroll_events_overshoot_id,
                                "[gtk] start_scroll_deceleration_cb");
     }
+}
+
+static gboolean
+scroll_controller_scroll (GtkEventControllerScroll *scroll,
+                          double                    delta_x,
+                          double                    delta_y,
+                          GtkScrolledWindow        *scrolled_window)
+{
+  GtkScrolledWindowPrivate *priv =
+    gtk_scrolled_window_get_instance_private (scrolled_window);
+
+  if (!priv->smooth_scroll)
+    scrolled_window_scroll (scrolled_window, delta_x, delta_y, scroll);
 
   return GDK_EVENT_STOP;
 }
@@ -2056,8 +2084,6 @@ gtk_scrolled_window_init (GtkScrolledWindow *scrolled_window)
                     G_CALLBACK (scroll_controller_scroll), scrolled_window);
   g_signal_connect (controller, "scroll-end",
                     G_CALLBACK (scroll_controller_scroll_end), scrolled_window);
-  g_signal_connect (controller, "decelerate",
-                    G_CALLBACK (scroll_controller_decelerate), scrolled_window);
   gtk_widget_add_controller (widget, controller);
 
   controller = gtk_event_controller_scroll_new (GTK_EVENT_CONTROLLER_SCROLL_BOTH_AXES |
@@ -2065,6 +2091,8 @@ gtk_scrolled_window_init (GtkScrolledWindow *scrolled_window)
   gtk_event_controller_set_propagation_phase (controller, GTK_PHASE_CAPTURE);
   g_signal_connect (controller, "scroll",
                     G_CALLBACK (captured_scroll_cb), scrolled_window);
+  g_signal_connect (controller, "decelerate",
+                    G_CALLBACK (scroll_controller_decelerate), scrolled_window);
   gtk_widget_add_controller (widget, controller);
 
   controller = gtk_event_controller_motion_new ();