[fix] Restore ability to override onMouseDown in node subclasses (#5079)

This fixes a regression where node subclasses could no longer override
the onMouseDown method. The issue was introduced when title button
support was added by assigning onMouseDown in the constructor, which
prevented proper method inheritance.

Changes:
- Remove onMouseDown assignment from LGraphNode constructor
- Move title button click detection to LGraphCanvas before calling node.onMouseDown
- This preserves title button functionality while allowing subclasses to override onMouseDown

Fixes #5073

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

src/lib/litegraph/src/LGraphCanvas.ts

@@ -2683,6 +2683,26 @@ export class LGraphCanvas
         this.processNodeDblClicked(node)
       }
 
+      // Check for title button clicks before calling onMouseDown
+      if (node.title_buttons?.length && !node.flags.collapsed) {
+        // pos contains the offset from the node's position, so we need to use node-relative coordinates
+        const nodeRelativeX = pos[0]
+        const nodeRelativeY = pos[1]
+
+        for (let i = 0; i < node.title_buttons.length; i++) {
+          const button = node.title_buttons[i]
+          if (
+            button.visible &&
+            button.isPointInside(nodeRelativeX, nodeRelativeY)
+          ) {
+            node.onTitleButtonClick(button, this)
+            // Set a no-op click handler to prevent fallback canvas dragging
+            pointer.onClick = () => {}
+            return
+          }
+        }
+      }
+
       // Mousedown callback - can block drag
       if (node.onMouseDown?.(e, pos, this)) {
         // Node handled the event (e.g., title button clicked)

src/lib/litegraph/src/LGraphNode.ts

@@ -728,34 +728,6 @@ export class LGraphNode
       error: this.#getErrorStrokeStyle,
       selected: this.#getSelectedStrokeStyle
     }
-
-    // Assign onMouseDown implementation
-    this.onMouseDown = (
-      // @ts-expect-error - CanvasPointerEvent type needs fixing
-      e: CanvasPointerEvent,
-      pos: Point,
-      canvas: LGraphCanvas
-    ): boolean => {
-      // Check for title button clicks (only if not collapsed)
-      if (this.title_buttons?.length && !this.flags.collapsed) {
-        // pos contains the offset from the node's position, so we need to use node-relative coordinates
-        const nodeRelativeX = pos[0]
-        const nodeRelativeY = pos[1]
-
-        for (let i = 0; i < this.title_buttons.length; i++) {
-          const button = this.title_buttons[i]
-          if (
-            button.visible &&
-            button.isPointInside(nodeRelativeX, nodeRelativeY)
-          ) {
-            this.onTitleButtonClick(button, canvas)
-            return true // Prevent default behavior
-          }
-        }
-      }
-
-      return false // Allow default behavior
-    }
   }
 
   /** Internal callback for subgraph nodes. Do not implement externally. */