[gtk+] zoom scrolling: avoid 'jumping back'

[Matthias Clasen <matthiasc src gnome org>]

commit 8726c6d5d239a342f0267d1c2bf0c90fb051d7fb
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Feb 23 06:42:17 2015 -0500

    zoom scrolling: avoid 'jumping back'
    
    We record the starting position on button press, but only
    start the zoom mode when the long press timeout kicks in.
    Depending on circumstances, this can cause a noticable jump.
    
    Avoid this by resetting the recorded starting position after
    the long press timeout. Suggested by Sebastian Keller.

 gtk/gtkrange.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtkrange.c b/gtk/gtkrange.c
index 404dc3a..bd95ee9 100644
--- a/gtk/gtkrange.c
+++ b/gtk/gtkrange.c
@@ -2437,7 +2437,12 @@ gtk_range_long_press_gesture_pressed (GtkGestureLongPress *gesture,
                                       gdouble              y,
                                       GtkRange            *range)
 {
-  update_zoom_state (range, TRUE);
+  if (!range->priv->zoom)
+    {
+      /* unset initial position so it can be calculated */
+      range->priv->slide_initial_slider_position = -1;
+      update_zoom_state (range, TRUE);
+    }
 }
 
 static void