Doing this will cause the player to fall into the shulker box, however when you close the shulker box the player snaps back to the top of the block.
Steps to reproduce:
Place a shulker box with any solid block 3 blocks above it
Stand on the shulker box and open it
[media]
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-158640, which is a report that has not been resolved yet.
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