The bug

This bug allows for players in survival to ascend infinitely with an Elytra without using fireworks, and without having any initial velocity. It also enables jumping extremely large horizontal distances (I was able to jump a distance of 278 blocks while staying above y=256 with slow falling and jump boost 2) in survival.

How to reproduce

1. Create an upwards bubble column that starts at any level and ends at the build height.
2. Create a platform to land on at the top of the bubble column.
3. Give yourself jump boost 2 (This isn't required to reproduce the bug, but it seems to make reproducing it more consistent).
4. Enter the bubble column from the bottom, and hold space all the way up until you are shot out the top. Move to the side so you land on the platform.
5. If you jump now, and stay above the build limit, your jumps should be much higher than normal, even with jump boost.
6. Equip an Elytra and start gliding, making sure to stay above the build limit. If you hold space, you will be able to ascend without limit. Note: You must keep your pitch level with the horizon. I'm not sure what the exact range is for pitch, but it works as long as you don't look straight up or down.

I have tested this in singleplayer and on multiplayer servers running Spigot 1.13-pre7. It works on both.