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

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
+       * image. The drop shadows are allowed to escape the nodes allocation if
-       * like boder and background color into account).
+       * 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 ();
     }
 }