Wood will expand and contract across the grain, and a wide panel made of solid wood could change width by a half of an inch, warping the door frame. By allowing the wood panel to float, it can expand and contract without damaging the door.
Put trimmed and cleaned green beans in the steamer basket, set over the boiling water, cover, and steam until tender to the bite, 3 to 5 minutes (depending, again, on how crisp or tender you like your green beans).
Bring a large saucepan of water to boiling. Add 1 tablespoon salt and the green beans. Cook beans for 3 to 4 minutes or until crisp-tender. Drain beans and immediately plunge them into ice water. Let the beans sit in ice water for 3 minutes or until cool.