Yes. Not only that, it should keep the submenu open when traveling inside this triangle (but close on timeout if mouse stays still inside it):

      +--------+
      | parent |
      |        +--------+
      |       /|submenu |
      |      / |        |
      |=====/==|        |
      |=====\==|        |
      |      \ |        |
      +------ \|        |
               +--------+

and close immediately when not inside the triangle.

Also, if in the parent there's another menu item with submenu somewhere on the path of mouse inside this triangle, ignore this item.

For best results, if mouse was inside the submenu for a moment, keep the submenu open until clicking away from it or moving over a different item in the parent menu. So many details!