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

diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index ffefb0f7d94365275348f70307188faf863ce495..d27d05c4327bda2618122295865dfb508d60ddf7 100644 (file)
--- 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 ();