c(new go.Point(mx, my));
          const part = myDiagram.findPartAt(point, true);
          onHighlight(part);
        }

        if (event.target.className === 'dropzone') {
          // Disallow a drop by returning before a call to preventDefault:
          return;
        }

        // Allow a drop on everything else
        event.preventDefault();
      },
      false
    );

    div.addEventListener(
      'dragleave',
      (event) => {
        // reset background of potential drop target
        if 