The title is misleading. I thought the bishop had to go out of some box. If I just saw the puzzle without the title I may have calculated a king move. However, moving the king to h6 is not thinking out of the box.
"Think outside the box"
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It allows you to look beyond a defined scope of relevance to find answers that would not exist otherwise.
Thinking outside the box can help to foster creativity and innovation, it often results in new and improved ideas.