commit 391fcf5868dac9aa28992cd62fae9cb104364835
Author: Cody Russell <cody@jhu.edu>
Date:   Sat Jan 14 14:49:14 2017 -0600

    Recompute viewport allocation when updating overlay scrollbars
    
    https://bugzilla.xamarin.com/show_bug.cgi?id=51375
    https://bugzilla.xamarin.com/show_bug.cgi?id=51382

diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index 3999d7d..0a2cb67 100644
--- a/gtk/gtkscrolledwindow.c
+++ b/gtk/gtkscrolledwindow.c
@@ -270,6 +270,8 @@ static void      gtk_scrolled_window_get_scroll_areas  (GtkScrolledWindow *scrol
                                                         GdkRectangle      *hslider_rect);
 static void  gtk_scrolled_window_update_scrollbars (GtkScrolledWindow *scrolled_window);
 
+static void  gtk_scrolled_window_compute_viewport_allocation (GtkScrolledWindow *scrolled_window);
+
 static void gtk_scrolled_window_overlay_scrollbars_changed (GtkSettings *settings,
                                                             GParamSpec  *arg,
                                                             gpointer     user_data);
@@ -1391,6 +1393,8 @@ gtk_scrolled_window_update_scrollbars (GtkScrolledWindow *scrolled_window)
   if (!priv->overlay_scrollbars || !gtk_widget_get_realized (widget))
     return;
 
+  gtk_scrolled_window_compute_viewport_allocation (scrolled_window);
+
   window = gtk_widget_get_window (gtk_widget_get_toplevel (widget));
   window_height = gdk_window_get_height (window);