Minecraft wiki entry for Shulker_Box, associated text: Like a chest, a shulker box requires a transparent area above to be opened, and it opens only if the hitbox of the top of the open shulker box does not intersect with another hitbox. This way, a shulker box can be opened with an upside-down slab or an upper trapdoor on top of it. It is also unique in that when opened, its hitbox expands to 1.5 blocks high/wide, physically pushing entities where it is facing.