[gtk+] zoom scrolling: Improve the previous fix

[Matthias Clasen <matthiasc src gnome org>]

commit c060d93e3dc8abea39b919ac61434219b4318f25
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Feb 23 07:28:40 2015 -0500

    zoom scrolling: Improve the previous fix
    
    As Sebastian pointed out, just resetting the initial slider
    position was an incomplete fix, because it does not cause the
    delta to be recomputed, which is important in this scenario,
    because you've likely travelled some distance over the slider
    before the long press kicks in.
    Instead, explicitly record both the slider position and the
    delta.

 gtk/gtkrange.c |   17 ++++++++++++++---
 1 files changed, 14 insertions(+), 3 deletions(-)
---
diff --git a/gtk/gtkrange.c b/gtk/gtkrange.c
index bd95ee9..277b78d 100644
--- a/gtk/gtkrange.c
+++ b/gtk/gtkrange.c
@@ -2437,10 +2437,21 @@ gtk_range_long_press_gesture_pressed (GtkGestureLongPress *gesture,
                                       gdouble              y,
                                       GtkRange            *range)
 {
-  if (!range->priv->zoom)
+  GtkRangePrivate *priv = range->priv;
+
+  if (!priv->zoom)
     {
-      /* unset initial position so it can be calculated */
-      range->priv->slide_initial_slider_position = -1;
+      if (priv->orientation == GTK_ORIENTATION_VERTICAL)
+        {
+          priv->slide_initial_slider_position = priv->slider.y;
+          priv->slide_initial_coordinate_delta = y - priv->slider.y;
+        }
+      else
+        {
+          priv->slide_initial_slider_position = priv->slider.x;
+          priv->slide_initial_coordinate_delta = x - priv->slider.x;
+        }
+
       update_zoom_state (range, TRUE);
     }
 }