Expectation:

1. When opening a shulker box with 2 blocks of air above it, the player standing on top of the box gets forced into a crouch.
2. When opening a shulker box with 1.5 blocks of air above it, the player (already crouching) gets forced into a crawl.
3. When opening a shulker box with 1 block of air above it, the player (already crawling) gets pushed out of the way or takes suffocation damage.

Reality:

In all cases, the player falls through the shulker box when it is fully opened, landing on the block it's on (or falling should there not be a block underneath it, the inventory of the box closing a short distance away (as expected)). Furthermore,

1. With two blocks of air the player is popped back up on top of the shulker box is closed. If a slab is moved above the player while the shulker box is opened, they will be pushed aside if possible. If not possible, various things happen. If a slab is moved 1.5 blocks above the shulker box, the player is stuck inside the box and able to jump, but not escape (Test 1). If 1 block above, the player is stuck similar to 1.5 blocks but unable to jump (Test 2). If .5 blocks above or directly on top of the slab, the player is popped back on top of the box but inside the slab (Test 3 and 4 respectively). If all but the .5 above the box is filled with blocks while this glitch is happening, the player will be trapped in the box with their head stuck in the top slab above the box, able to see through any blocks above them (Test 5). If both blocks are filled, the player suffocates in the block above the shulkerbox.
2. From a crouched position, the player will be popped back into a crouched position when the box is closed. However, if a slab 1 block above the shulker is pushed above the player, it acts like Test 2. If a slab is moved .5 blocks above the shulker, the player remains within the box and is inside the slab as well, but looks to be crouching in first person (Test 6 and 6.2). If a slab is moved directly on top of the shulker box, the player pops on top of the shulker box, standing inside both the slab and the slab 1.5 blocks above the box but will appear to be crouching in first person (Test 7 and 7.2). 
3. When crawling, the player gets trapped inside the shulker box regardless of if slabs are moved into the space above the box.