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!

Not difficult at all. For a simple fix, one just needs to add a timeout between receiving the mouseOut event and hiding the menu. OS X appears to do something slightly more complicated; it hides the menu immediately if you mouse out away from the submenu, but keeps the menu open for about one second if you mouse out towards it.

Its not difficult at all actually.

Just requires a timeout for the onmouseover event. jQuery's hover intent would work wonders.

Basically show the submenu onmouseover of parent and if no mouseover is detected on the submenu within 400ms => hide the submenu.

It gives the user a slight bit of time to move the mouse to the submenu rather than relying on perfect mouse movements.