st-scroll-view: Add :overlay-scrollbars property

If enabled, scrollbars take away from the allocation given to the
view's content. This is usually preferrable to painting the bars on
top of the content, but there are exceptions, for instance when the
content needs to be centered with regard to the view as a whole.
Add a :overlay-scrollbars property to account for those cases.

https://bugzilla.gnome.org/show_bug.cgi?id=694261

src/st/st-scroll-view.c

@@ -101,6 +101,7 @@ struct _StScrollViewPrivate
   gboolean      row_size_set : 1;
   gboolean      column_size_set : 1;
   guint         mouse_scroll : 1;
+  guint         overlay_scrollbars : 1;
   guint         hscrollbar_visible : 1;
   guint         vscrollbar_visible : 1;
 };
@@ -115,6 +116,7 @@ enum {
   PROP_HSCROLLBAR_VISIBLE,
   PROP_VSCROLLBAR_VISIBLE,
   PROP_MOUSE_SCROLL,
+  PROP_OVERLAY_SCROLLBARS,
 };
 
 static void
@@ -148,6 +150,9 @@ st_scroll_view_get_property (GObject    *object,
     case PROP_MOUSE_SCROLL:
       g_value_set_boolean (value, priv->mouse_scroll);
       break;
+    case PROP_OVERLAY_SCROLLBARS:
+      g_value_set_boolean (value, priv->overlay_scrollbars);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);
     }
@@ -212,6 +217,10 @@ st_scroll_view_set_property (GObject      *object,
       st_scroll_view_set_mouse_scrolling (self,
                                           g_value_get_boolean (value));
       break;
+    case PROP_OVERLAY_SCROLLBARS:
+      st_scroll_view_set_overlay_scrollbars (self,
+                                             g_value_get_boolean (value));
+      break;
     case PROP_HSCROLLBAR_POLICY:
       st_scroll_view_set_policy (self,
                                  g_value_get_enum (value),
@@ -376,14 +385,14 @@ st_scroll_view_get_preferred_width (ClutterActor *actor,
       account_for_vscrollbar = FALSE;
       break;
     case GTK_POLICY_ALWAYS:
-      account_for_vscrollbar = TRUE;
+      account_for_vscrollbar = !priv->overlay_scrollbars;
       break;
     case GTK_POLICY_AUTOMATIC:
       /* For automatic scrollbars, we always request space for the vertical
        * scrollbar; we won't know whether we actually need one until our
        * height is assigned in allocate().
        */
-      account_for_vscrollbar = TRUE;
+      account_for_vscrollbar = !priv->overlay_scrollbars;
       break;
     }
 
@@ -448,14 +457,14 @@ st_scroll_view_get_preferred_height (ClutterActor *actor,
       account_for_hscrollbar = FALSE;
       break;
     case GTK_POLICY_ALWAYS:
-      account_for_hscrollbar = TRUE;
+      account_for_hscrollbar = !priv->overlay_scrollbars;
       break;
     case GTK_POLICY_AUTOMATIC:
       /* For automatic scrollbars, we always request space for the horizontal
        * scrollbar; we won't know whether we actually need one until our
        * width is assigned in allocate().
        */
-      account_for_hscrollbar = TRUE;
+      account_for_hscrollbar = !priv->overlay_scrollbars;
       break;
     }
 
@@ -626,14 +635,15 @@ st_scroll_view_allocate (ClutterActor          *actor,
       clutter_actor_allocate (priv->hscroll, &child_box, flags);
     }
 
-  /* In case the scrollbar policy is NEVER, we don't trim the content
+  /* In case the scrollbar policy is NEVER or scrollbars should be
-   * box allocation by the scrollbar size.
+   * overlayed, we don't trim the content box allocation by the
+   * scrollbar size.
    * Fold this into the scrollbar sizes to simplify the rest of the
    * computations.
    */
-  if (priv->hscrollbar_policy == GTK_POLICY_NEVER)
+  if (priv->hscrollbar_policy == GTK_POLICY_NEVER || priv->overlay_scrollbars)
     sb_height = 0;
-  if (priv->vscrollbar_policy == GTK_POLICY_NEVER)
+  if (priv->vscrollbar_policy == GTK_POLICY_NEVER || priv->overlay_scrollbars)
     sb_width = 0;
 
   /* Child */
@@ -828,6 +838,14 @@ st_scroll_view_class_init (StScrollViewClass *klass)
                                    PROP_MOUSE_SCROLL,
                                    pspec);
 
+  pspec = g_param_spec_boolean ("overlay-scrollbars",
+                                "Use Overlay Scrollbars",
+                                "Overlay scrollbars over the content",
+                                FALSE,
+                                G_PARAM_READWRITE);
+  g_object_class_install_property (object_class,
+                                   PROP_OVERLAY_SCROLLBARS,
+                                   pspec);
 }
 
 static void
@@ -1090,6 +1108,49 @@ st_scroll_view_get_mouse_scrolling (StScrollView *scroll)
   return priv->mouse_scroll;
 }
 
+/**
+ * st_scroll_view_set_overlay_scrollbars:
+ * @scroll: A #StScrollView
+ * @enabled: Whether to enable overlay scrollbars
+ *
+ * Sets whether scrollbars are painted on top of the content.
+ */
+void
+st_scroll_view_set_overlay_scrollbars (StScrollView *scroll,
+                                       gboolean      enabled)
+{
+  StScrollViewPrivate *priv;
+
+  g_return_if_fail (ST_IS_SCROLL_VIEW (scroll));
+
+  priv = ST_SCROLL_VIEW (scroll)->priv;
+
+  if (priv->overlay_scrollbars != enabled)
+    {
+      priv->overlay_scrollbars = enabled;
+      g_object_notify (G_OBJECT (scroll), "overlay-scrollbars");
+      clutter_actor_queue_relayout (CLUTTER_ACTOR (scroll));
+    }
+}
+
+/**
+ * st_scroll_view_get_overlay_scrollbars:
+ * @scroll: A #StScrollView
+ *
+ * Gets the value set by st_scroll_view_set_overlay_scrollbars().
+ */
+gboolean
+st_scroll_view_get_overlay_scrollbars (StScrollView *scroll)
+{
+  StScrollViewPrivate *priv;
+
+  g_return_val_if_fail (ST_IS_SCROLL_VIEW (scroll), FALSE);
+
+  priv = ST_SCROLL_VIEW (scroll)->priv;
+
+  return priv->overlay_scrollbars;
+}
+
 /**
  * st_scroll_view_set_policy:
  * @scroll: A #StScrollView

src/st/st-scroll-view.h

@@ -80,6 +80,10 @@ void          st_scroll_view_set_mouse_scrolling (StScrollView *scroll,
                                                   gboolean      enabled);
 gboolean      st_scroll_view_get_mouse_scrolling (StScrollView *scroll);
 
+void          st_scroll_view_set_overlay_scrollbars (StScrollView *scroll,
+                                                     gboolean      enabled);
+gboolean      st_scroll_view_get_overlay_scrollbars (StScrollView *scroll);
+
 void          st_scroll_view_set_policy          (StScrollView   *scroll,
                                                   GtkPolicyType   hscroll,
                                                   GtkPolicyType   vscroll);

tests/interactive/scroll-view-sizing.js

@@ -339,6 +339,10 @@ function test() {
     let vfade = new St.Button({ label: 'No', style: 'text-decoration: underline; color: #4444ff;' });
     fadeBox.add(vfade);
 
+    fadeBox.add(new St.Label({ text: 'Overlay scrollbars: '}));
+    let overlay = new St.Button({ label: 'No', style: 'text-decoration: underline; color: #4444ff;' });
+    fadeBox.add(overlay);
+
     function togglePadding(button) {
         switch(button.label) {
         case 'No':
@@ -388,6 +392,20 @@ function test() {
     vfade.connect('clicked', function() { toggleFade(vfade); });
     toggleFade(vfade);
 
+    function toggleOverlay(button) {
+        switch(button.label) {
+        case 'No':
+	    button.label = 'Yes';
+	    break;
+        case 'Yes':
+	    button.label = 'No';
+	    break;
+        }
+        scrollView.overlay_scrollbars = (button.label == 'Yes');
+    }
+
+    overlay.connect('clicked', function() { toggleOverlay(overlay); });
+
     UI.main(stage);
 }
 test();