click on submenu to freeze it

with apologies if this has been suggested previously (if so please
point me to the entry): I think it would be really good if there were
an add-on which froze the "upstream" state of nested menus upon
clicking on a ">" item in the currently-displayed menu level. I use
tiny menus and have a whole lot of nested bookmark menus, and getting
around them can be a tad painful if you're not very precise with the
mouse. what I'm suggesting is that if, for example,  you've clicked
through to here:

menu
  submenu1>
  submenu2> -- subsubmenu1>
  submenu3>     subsubmenu2> --  subsubsubmenu1>
  submenu4>     subsubmenu3>      subsubsubmenu2>
  submenu5>

then if you *click* on "subsubmenu2", subsequent moves of the mouse
which happen to fall over other submenus don't cause those menus to
expand up. To reverse the frozen state you would just click once again
on subsubmenu2.