if the user select the checkbox of a root forum, all subforum should be auto. selected/deselected in the search field
(https://www.simplemachines.org/community/proxy.php?request=http%3A%2F%2Fimg523.imageshack.us%2Fimg523%2F1411%2Fsddsdsdsak0.png&hash=d11c39247c8797ba26728d3d48c389879eb54f29)
that is a good idea!
You can currently do it category-wise by clicking the category link.
I don't know if that helps in the short-term.