gtk: Allow hiding the trough/slider in GtkScale

When setting a GtkRange's upper and lower values to the same
value, the slider will not be drawn any more.

https://bugzilla.gnome.org/show_bug.cgi?id=549720

gtk/gtkrange.c

@@ -1280,7 +1280,7 @@ gtk_range_set_range (GtkRange *range,
   gdouble value;
   
   g_return_if_fail (GTK_IS_RANGE (range));
-  g_return_if_fail (min < max);
+  g_return_if_fail (min <= max);
 
   priv = range->priv;
 
@@ -2016,11 +2016,16 @@ gtk_range_draw (GtkWidget      *widget,
   gint focus_line_width = 0;
   gint focus_padding = 0;
   gboolean touchscreen;
+  gboolean draw_trough = TRUE;
 
   g_object_get (gtk_widget_get_settings (widget),
                 "gtk-touchscreen-mode", &touchscreen,
                 NULL);
 
+  if (GTK_IS_SCALE (widget) &&
+      priv->adjustment->upper == priv->adjustment->lower)
+    draw_trough = FALSE;
+
   style = gtk_widget_get_style (widget);
   if (gtk_widget_get_can_focus (GTK_WIDGET (range)))
     gtk_widget_style_get (GTK_WIDGET (range),
@@ -2112,6 +2117,7 @@ gtk_range_draw (GtkWidget      *widget,
             }
 	}
 
+      if (draw_trough)
         {
 	  gint trough_change_pos_x = width;
 	  gint trough_change_pos_y = height;
@@ -2147,6 +2153,17 @@ gtk_range_draw (GtkWidget      *widget,
                          width - trough_change_pos_x,
                          height - trough_change_pos_y);
         }
+      else
+        {
+          gtk_paint_box (style, cr,
+                         sensitive ? GTK_STATE_ACTIVE : GTK_STATE_INSENSITIVE,
+                         GTK_SHADOW_IN,
+                         GTK_WIDGET (range),
+                         "trough-upper",
+                         x, y,
+                         width,
+                         height);
+        }
 
       if (priv->show_fill_level &&
           priv->adjustment->upper - priv->adjustment->page_size -
@@ -2236,6 +2253,7 @@ gtk_range_draw (GtkWidget      *widget,
   gdk_cairo_rectangle (cr, &priv->slider);
   cairo_clip (cr);
 
+  if (draw_trough)
     {
       gtk_paint_slider (style,
                         cr,

gtk/gtkscale.c

@@ -55,6 +55,11 @@
  * To detect changes to the value, you would normally use the
  * #GtkRange::value-changed signal.
  *
+ * Note that using the same upper and lower bounds for the #GtkScale (through
+ * the #GtkRange methods) will hide the slider itself. This is useful for
+ * applications that want to show an undeterminate value on the scale, without
+ * changing the layout of the application (such as movie or music players).
+ *
  * <refsect2 id="GtkScale-BUILDER-UI"><title>GtkScale as GtkBuildable</title>
  * GtkScale supports a custom &lt;marks&gt; element, which
  * can contain multiple &lt;mark&gt; elements. The "value" and "position"

tests/testscale.c

@@ -20,6 +20,16 @@
 
 #include <gtk/gtk.h>
 
+static void
+show_trough_toggled (GtkToggleButton *button,
+                     GtkScale        *scale)
+{
+  gboolean value;
+
+  value = gtk_toggle_button_get_active (button);
+  gtk_range_set_range (GTK_RANGE (scale), 0., value ? 100.0 : 0.);
+}
+
 int main (int argc, char *argv[])
 {
   GtkWidget *window;
@@ -27,6 +37,7 @@ int main (int argc, char *argv[])
   GtkWidget *box2;
   GtkWidget *frame;
   GtkWidget *scale;
+  GtkWidget *toggle;
   gdouble marks[3] = { 0.0, 50.0, 100.0 };
   const gchar *labels[3] = { 
     "<small>Left</small>", 
@@ -96,6 +107,19 @@ int main (int argc, char *argv[])
   gtk_container_add (GTK_CONTAINER (frame), scale);
   gtk_box_pack_start (GTK_BOX (box), frame, FALSE, FALSE, 0);
 
+  frame = gtk_frame_new ("Show/hide trough");
+  box2 = gtk_box_new (GTK_ORIENTATION_VERTICAL, 5);
+  scale = gtk_scale_new_with_range (GTK_ORIENTATION_HORIZONTAL,
+                                    0, 100, 1);
+  gtk_box_pack_start (GTK_BOX (box2), scale, TRUE, TRUE, 0);
+  toggle = gtk_toggle_button_new_with_label ("Show slider trough");
+  gtk_toggle_button_set_active (GTK_TOGGLE_BUTTON (toggle), TRUE);
+  g_signal_connect (G_OBJECT (toggle), "toggled",
+                    G_CALLBACK (show_trough_toggled), scale);
+  gtk_box_pack_start (GTK_BOX (box2), toggle, TRUE, TRUE, 0);
+  gtk_container_add (GTK_CONTAINER (frame), box2);
+  gtk_box_pack_start (GTK_BOX (box), frame, FALSE, FALSE, 0);
+
   box2 = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 5);
   gtk_box_pack_start (GTK_BOX (box), box2, TRUE, TRUE, 0);