Canselable event not canseled.

I have a xulrunner application(c++) with tree widget(custom tree view).
I'd like to change the behavior of tree widget when i press "right" arrow.
Not reasonable Mozilla's way to open container instead of go to the right neighbour cell(selection type = cell).

if i catch "keydown" event, "canselable"-attribute is equal to "true",
but PreventDefault() do not prevent default action(the container is opening).

Of course I can check in ToggleOpenState() function whether "right arrow" is pressed in first column and then cansel ToggleOpenState(). 
But may be there is a simpler way? And i'd like to understand more Mozilla's internals.