ssd: attach LAB_NODE_SSD_ROOT to ssd->tree

This doesn't change any behaviors.

Attaching LAB_NODE_NONE node-descriptor to ssd->tree looks strange, this
patch uses new LAB_NODE_SSD_ROOT instead. The node-descriptor attached to
ssd->tree is needed for get_cursor_context() to detect cursor hovering on
borders/extents.

I also updated get_cursor_context() to make my intent clearer.

include/common/node-type.h

@@ -59,6 +59,12 @@ enum lab_node_type {
 	LAB_NODE_LAYER_POPUP,
 	LAB_NODE_SESSION_LOCK_SURFACE,
 	LAB_NODE_IME_POPUP,
+
+	/*
+	 * translated to LAB_CORNER_* or LAB_BORDER* by
+	 * ssd_get_resizing_type()
+	 */
+	LAB_NODE_SSD_ROOT,
 };
 
 enum lab_node_type node_type_parse(const char *context);

include/ssd-internal.h

@@ -17,7 +17,7 @@ struct ssd_state_title_width {
  * type of each node (enum lab_node_type, stored in the node_descriptor
  * attached to the wlr_scene_node).
  *
- * ssd->tree (LAB_NODE_NONE)
+ * ssd->tree (LAB_NODE_SSD_ROOT)
  * +--titlebar (LAB_NODE_TITLEBAR)
  * |  +--inactive
  * |  |  +--background bar

src/desktop.c

@@ -331,31 +331,46 @@ get_cursor_context(struct server *server)
 				ret.node = node;
 				ret.type = LAB_NODE_MENUITEM;
 				return ret;
-			default:
+			case LAB_NODE_BUTTON_FIRST...LAB_NODE_BUTTON_LAST:
-				/*
+			case LAB_NODE_SSD_ROOT:
-				 * All other node descriptors (buttons, title,
+			case LAB_NODE_TITLE:
-				 * etc.) should have an associated view.
+			case LAB_NODE_TITLEBAR:
-				 */
-				if (!desc->view) {
-					wlr_log(WLR_ERROR, "cursor not on any view "
-						"(node type %d)", desc->type);
-					return ret;
-				}
-
 				ret.node = node;
 				ret.view = desc->view;
+				/*
+				 * A node_descriptor attached to a ssd part
+				 * must have an associated view.
+				 */
+				assert(ret.view);
 
-				/* Detect mouse contexts like Top, Left and TRCorner */
+				/*
+				 * When cursor is on the ssd border or extents,
+				 * desc->type is usually LAB_NODE_SSD_ROOT.
+				 * But desc->type can also be LAB_NODE_TITLEBAR
+				 * when cursor is on the curved border at the
+				 * titlebar.
+				 *
+				 * ssd_get_resizing_type() overwrites both of
+				 * them with LAB_NODE_{BORDER,CORNER}_* node
+				 * types, which are mapped to mouse contexts
+				 * like Left and TLCorner.
+				 */
 				ret.type = ssd_get_resizing_type(ret.view->ssd, cursor);
 				if (ret.type == LAB_NODE_NONE) {
 					/*
-					 * Otherwise, detect mouse contexts like
+					 * If cursor is not on border/extents,
-					 * Title, Titlebar and Iconify
+					 * just use desc->type which should be
+					 * mapped to mouse contexts like Title,
+					 * Titlebar and Iconify.
 					 */
 					ret.type = desc->type;
 				}
 
 				return ret;
+			default:
+				/* Other node types are not attached a scene node */
+				wlr_log(WLR_ERROR, "unexpected node type: %d", desc->type);
+				break;
 			}
 		}
 

src/ssd/ssd.c

@@ -146,8 +146,14 @@ ssd_create(struct view *view, bool active)
 
 	ssd->view = view;
 	ssd->tree = wlr_scene_tree_create(view->scene_tree);
+
+	/*
+	 * Attach node_descriptor to the root node so that get_cursor_context()
+	 * detect cursor hovering on borders and extents.
+	 */
 	node_descriptor_create(&ssd->tree->node,
-		LAB_NODE_NONE, view, /*data*/ NULL);
+		LAB_NODE_SSD_ROOT, view, /*data*/ NULL);
+
 	wlr_scene_node_lower_to_bottom(&ssd->tree->node);
 	ssd->titlebar.height = view->server->theme->titlebar_height;
 	ssd_shadow_create(ssd);