Add press and hold support to set zoom scroll mode

Triggering zoom scroll mode by Shift click was too much
of an easter egg. It also requires using keyboard and
mouse together, which is hard to do for many users.

Instead, we now trigger zoom scroll mode by click-and-hold
(or touch-and-hold).

https://bugzilla.gnome.org/show_bug.cgi?id=704703

gtk/gtkrange.c

@@ -36,6 +36,7 @@
 #include "gtkmain.h"
 #include "gtkmarshalers.h"
 #include "gtkorientableprivate.h"
+#include "gtkpressandholdprivate.h"
 #include "gtkprivate.h"
 #include "gtkscale.h"
 #include "gtkscrollbar.h"
@@ -62,6 +63,7 @@
 #define UPDATE_DELAY        300  /* Delay for queued update */
 #define TIMEOUT_INITIAL     500
 #define TIMEOUT_REPEAT      50
+#define ZOOM_HOLD_TIME      500
 
 typedef struct _GtkRangeStepTimer GtkRangeStepTimer;
 
@@ -142,6 +144,8 @@ struct _GtkRangePrivate
 
   /* Whether we're doing fine adjustment */
   guint zoom                   : 1;
+  guint zoom_set               : 1;
+  GtkPressAndHold *press_and_hold;
 
   /* Fill level */
   guint show_fill_level        : 1;
@@ -1499,6 +1503,8 @@ gtk_range_destroy (GtkWidget *widget)
 
   gtk_range_remove_step_timer (range);
 
+  g_clear_object (&priv->press_and_hold);
+
   if (priv->adjustment)
     {
       g_signal_handlers_disconnect_by_func (priv->adjustment,
@@ -2346,6 +2352,33 @@ range_grab_add (GtkRange      *range,
     gtk_widget_queue_draw (GTK_WIDGET (range));
 }
 
+static void
+update_zoom_state (GtkRange *range,
+                   gboolean  enabled)
+{
+  GtkStyleContext *context;
+
+  context = gtk_widget_get_style_context (GTK_WIDGET (range));
+
+  if (enabled)
+    gtk_style_context_add_class (context, "fine-tune");
+  else
+    gtk_style_context_remove_class (context, "fine-tune");
+  gtk_widget_queue_draw (GTK_WIDGET (range));
+
+  range->priv->zoom = enabled;
+}
+
+static void
+update_zoom_set (GtkRange *range,
+                 gboolean  zoom_set)
+{
+  if (zoom_set)
+    g_clear_object (&range->priv->press_and_hold);
+
+  range->priv->zoom_set = zoom_set;
+}
+
 static void
 range_grab_remove (GtkRange *range)
 {
@@ -2368,7 +2401,8 @@ range_grab_remove (GtkRange *range)
       location != MOUSE_OUTSIDE)
     gtk_widget_queue_draw (GTK_WIDGET (range));
 
-  priv->zoom = FALSE;
+  update_zoom_state (range, FALSE);
+  update_zoom_set (range, FALSE);
 }
 
 static GtkScrollType
@@ -2511,6 +2545,16 @@ gtk_range_key_press (GtkWidget   *widget,
   return GTK_WIDGET_CLASS (gtk_range_parent_class)->key_press_event (widget, event);
 }
 
+static void
+hold_action (GtkPressAndHold *pah,
+             gint             x,
+             gint             y,
+             GtkRange        *range)
+{
+  update_zoom_state (range, TRUE);
+  update_zoom_set (range, TRUE);
+}
+
 static gint
 gtk_range_button_press (GtkWidget      *widget,
 			GdkEventButton *event)
@@ -2657,7 +2701,32 @@ gtk_range_button_press (GtkWidget      *widget,
         {
           /* Shift-click in the slider = fine adjustment */
           if (event->state & GDK_SHIFT_MASK)
-            priv->zoom = TRUE;
+            {
+              update_zoom_state (range, TRUE);
+              update_zoom_set (range, TRUE);
+            }
+          else
+            {
+              if (!priv->press_and_hold)
+                {
+                  gint drag_threshold;
+
+                  g_object_get (gtk_widget_get_settings (widget),
+                                "gtk-dnd-drag-threshold", &drag_threshold,
+                                NULL);
+                  priv->press_and_hold = gtk_press_and_hold_new ();
+
+                  g_object_set (priv->press_and_hold,
+                                "drag-threshold", drag_threshold,
+                                "hold-time", ZOOM_HOLD_TIME,
+                                NULL);
+
+                  g_signal_connect (priv->press_and_hold, "hold",
+                                    G_CALLBACK (hold_action), range);
+                }
+
+              gtk_press_and_hold_process_event (priv->press_and_hold, (GdkEvent *)event);
+            }
         }
 
       if (priv->orientation == GTK_ORIENTATION_VERTICAL)
@@ -2803,7 +2872,11 @@ gtk_range_button_release (GtkWidget      *widget,
       priv->grab_button == event->button)
     {
       if (priv->grab_location == MOUSE_SLIDER)
-        update_slider_position (range, priv->mouse_x, priv->mouse_y);
+        {
+          update_slider_position (range, priv->mouse_x, priv->mouse_y);
+          if (priv->press_and_hold)
+            gtk_press_and_hold_process_event (priv->press_and_hold, (GdkEvent *)event);
+        }
 
       stop_scrolling (range);
       
@@ -2905,7 +2978,12 @@ gtk_range_motion_notify (GtkWidget      *widget,
     gtk_widget_queue_draw (widget);
 
   if (priv->grab_location == MOUSE_SLIDER)
-    update_slider_position (range, event->x, event->y);
+    {
+      if (!priv->zoom_set && priv->press_and_hold != NULL)
+        gtk_press_and_hold_process_event (priv->press_and_hold, (GdkEvent *)event);
+
+      update_slider_position (range, event->x, event->y);
+    }
 
   /* We handled the event if the mouse was in the range_rect */
   return priv->mouse_location != MOUSE_OUTSIDE;