matches.add(src);
    } else {
      matches.delete(src);
    }
    toggleClass(src, 'hilite', on);
    return true;
  }

  // Display action menu (triggered by right-click on a frame)
  function showActionMenu(e, box) {
    if (box.src == 0) return; // No action menu for root
    e.preventDefault(); // Disable browser context menu
    const src = stacks.Sources[box.src];