Although hoppers do not visually take up a whole block, their hitbox does, which is sometimes annoying when you want to place something behind a hopper.
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I wish too Mojang would change the hopper hitbox to act like the stairs' hitbox. But they won't.
Duplicate of MC-6435 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.