StThemeNodeDrawing: clip background image shadow to outline

When drawing the background image shadow, we need to clip it
to the node's background color, gradient, or borders if present.

If the background color is transparent, and there aren't any
borders, then we don't clip the shadow since there is nothing
to confine it.

https://bugzilla.gnome.org/show_bug.cgi?id=636976

diff --git a/src/st/st-theme-node-drawing.c b/src/st/st-theme-node-drawing.c
index feea5fc09e6843559fd13ffa4e7b3bfc615850c8..9ffb7bb9cf2214d7179204befdeecd9968244a1f 100644 (file)
--- a/src/st/st-theme-node-drawing.c
+++ b/src/st/st-theme-node-drawing.c
@@ -427,6 +427,30 @@ get_arbitrary_border_color (StThemeNode   *node,
     st_theme_node_get_border_color (node, ST_SIDE_TOP, color);
 }
 
+static gboolean
+st_theme_node_has_visible_outline (StThemeNode *node)
+{
+  if (node->background_color.alpha > 0)
+    return TRUE;
+
+  if (node->background_gradient_end.alpha > 0)
+    return TRUE;
+
+  if (node->border_radius[ST_CORNER_TOPLEFT] > 0 ||
+      node->border_radius[ST_CORNER_TOPRIGHT] > 0 ||
+      node->border_radius[ST_CORNER_BOTTOMLEFT] > 0 ||
+      node->border_radius[ST_CORNER_BOTTOMRIGHT] > 0)
+    return TRUE;
+
+  if (node->border_width[ST_SIDE_TOP] > 0 ||
+      node->border_width[ST_SIDE_LEFT] > 0 ||
+      node->border_width[ST_SIDE_RIGHT] > 0 ||
+      node->border_width[ST_SIDE_BOTTOM] > 0)
+    return TRUE;
+
+  return FALSE;
+}
+
 static CoglHandle
 st_theme_node_render_gradient (StThemeNode *node)
 {
@@ -1308,6 +1332,16 @@ st_theme_node_paint (StThemeNode           *node,
       ClutterActorBox background_box;
 
       get_background_position (node, &allocation, &background_box);
+      gboolean has_visible_outline;
+
+      /* If the background doesn't have a border or opaque background,
+       * then we let its background image shadows leak out, but other
+       * wise we clip it.
+       */
+      has_visible_outline = st_theme_node_has_visible_outline (node);
+
+      if (has_visible_outline)
+        cogl_clip_push_rectangle (allocation.x1, allocation.y1, allocation.x2, allocation.y2);
 
       /* CSS based drop shadows
        *
@@ -1318,8 +1352,9 @@ st_theme_node_paint (StThemeNode           *node,
        * multiple shadows and allow for a more liberal placement of the color
        * parameter - its interpretation defers significantly in that the shadow's
        * shape is not determined by the bounding box, but by the CSS background
-       * image (we could exend this in the future to take other CSS properties
-       * like boder and background color into account).
+       * image. The drop shadows are allowed to escape the nodes allocation if
+       * there is nothing (like a border, or the edge of the background color)
+       * to logically confine it.
        */
       if (node->background_shadow_material != COGL_INVALID_HANDLE)
         _st_paint_shadow_with_opacity (node->background_image_shadow,
@@ -1328,6 +1363,9 @@ st_theme_node_paint (StThemeNode           *node,
                                        paint_opacity);
 
       paint_material_with_opacity (node->background_material, &background_box, paint_opacity);
+
+      if (has_visible_outline)
+        cogl_clip_pop ();
     }
 }