Prioritize scrolling a GtkScrolledWindow over its child widgets

Scrolling a GtkScrolledWindow should have precedence
over child widgets that also react to scrolling
like GtkComboBox, GtkScale and GtkSpinButton,
because otherwise when you're in the middle of
scrolling the window you can involuntarily
scroll over the widgets and change its values.

This problem can be seen in applications like
pavucontrol, gnome-control-center audio page,
gnome-tweaks, devhelp prefs, and so on.

However this patch allows to scroll the child
widgets when the GtkScrolledWindow itself cannot
be scrolled (i.e. when no scrollbars are shown
because window height exceeds the content).

Fixes issue #3092

gtk/gtkscrolledwindow.c

@@ -408,6 +408,10 @@ static void scrolled_window_scroll (GtkScrolledWindow        *scrolled_window,
                                     double                    delta_x,
                                     double                    delta_y,
                                     GtkEventControllerScroll *scroll);
+static gboolean scroll_controller_scroll (GtkEventControllerScroll *scroll,
+                                          double                    delta_x,
+                                          double                    delta_y,
+                                          GtkScrolledWindow        *scrolled_window);
 
 static guint signals[LAST_SIGNAL] = {0};
 static GParamSpec *properties[NUM_PROPERTIES];
@@ -1229,6 +1233,19 @@ get_wheel_detent_scroll_step (GtkScrolledWindow *sw,
   return scroll_step;
 }
 
+/* Returns whether @window can currently be scrolled
+ * i.e. the scrollbars can move because the content excedes the page_size */
+static gboolean
+content_can_be_scrolled (GtkScrolledWindow *window)
+{
+  GtkAdjustment *vadj;
+  gdouble upper, page_size;
+
+  vadj = gtk_scrolled_window_get_vadjustment (window);
+  g_object_get (vadj, "upper", &upper, "page_size", &page_size, NULL);
+  return !G_APPROX_VALUE ((upper - page_size), 0.0, DBL_EPSILON);
+}
+
 static gboolean
 captured_scroll_cb (GtkEventControllerScroll *scroll,
                     double                    delta_x,
@@ -1246,6 +1263,12 @@ captured_scroll_cb (GtkEventControllerScroll *scroll,
       return GDK_EVENT_STOP;
     }
 
+  if (content_can_be_scrolled (scrolled_window))
+    {
+      scroll_controller_scroll (scroll, delta_x, delta_y, scrolled_window);
+      return GDK_EVENT_STOP;
+    }
+
   return GDK_EVENT_PROPAGATE;
 }