By placing your character underneath a upper half block such as a slab or stair whilst under a bed and jumping, the player remains in the air for the amount of time as if there is no barrier preventing a jump into the air. However, the player should return to the ground at the same pace as if one was to jump in a two high space. This behavior does not occur on slime blocks. Bug started at version 1.12.

To recreate:

1. Open a world
2. Place down bed
3. Create a 3 block pillar behind bed
4. Place a upper half block (stair or slab) on top of the bed, on the third block of the pillar.
5. Jump on bed whilst underneath the half block.

(Sorry for the recording notification in the video, I just started using their software)