Opacity/Placement

Writing a draft for my detailed Transparent Blocks Effects page. Any feedback is welcome! This page is very WIP!

Doors and Trapdoors
Doors and trapdoors behave in unique ways. The flat side of each block acts as an opaque surface, allowing anything to be placed on it. However, all other sides act as transparent blocks. Because of this, many blocks that can be placed on doors and trapdoors will not stay in the door or trapdoor is opened or closed.

Doors
Signs can be placed anywhere on a door and will never be destroyed unless the door itself is destroyed. Cakes can be placed on the tops of doors and will not be destroyed when the door opens and closes. All other blocks will be destroyed when the door changes states.

Trapdoors
Signs can be placed anywhere on a trapdoor and will never be destroyed unless the trapdoor itself is destroyed. Cakes can be placed on the tops of trapdoors and will not be destroyed when the trapdoor opens and closes. Rails, redstone dust, redstone repeaters, and redstone comparatorscan only be placed on a closed trapdoor in the top half of a block, but will not be destroyed if the trapdoor is opened. All other blocks will be destroyed when the trapdoor changes states.

Shulker Boxes
Shulker boxes typically act as a transparent block in that they allow everything to be placed on them. However, many of these blocks will be destroyed when the shulker box is opened. This behavior is also dependent on the rotation of the shulker box.

Upwards facing shulker boxes
Upwards facing shulker boxes do not destroy most transparent blocks placed on their tops when opened. However, some blocks stop the opening of the box.

While most blocks placed on top of upwards facing shulker boxes, the same is not true for blocks placed on the sides. Signs are not destroyed, but the following blocks are.

Sideways facing shulker boxes
Sideways facing shulker boxes tend to destroy most transparent blocks when opened. However, some are not destroyed.
 * (Only when on top)
 * (Only when on top)
 * (Only when on top)
 * (Only when on top)

Downwards facing shulker boxes
Downwards facing shulker boxes do not destroy any blocks placed on their tops when opened. However, they do destroy some when placed on the sides.