The bug

Infinite upward elytra flight is possible due to a potential oversight with water and not resetting upward velocity at world height.

This bug affects Minecraft Java Edition versions 1.13 pre9 through 19w40a.

To reproduce

Here's a video of the bug shown in detail, with explanation.

1. Place at least one water source at the max world height and add some blocks adjacent to it.
2. Submerge yourself in said water; it doesn't need to be two blocks deep, you could just put your lower half in it.
3. Hold the jump key to swim up out of the water onto the adjacent blocks. At this point, you should notice that, if you jump, you get a sort of slow-falling effect.
4. Put on an Elytra, then jump and activate the Elytra.
5. Look in a mostly horizontal direction and continue to hold jump. You should see that you continue to go upwards. If you go in 3rd person mode, this is made even more obvious.

Other notes

1. If you set your pitch to 35.4 (or other close values to that) either manually or by doing /tp @s ~ ~ ~ ~ 35.4 and continue to hold jump, you will notice that, eventually, you can completely stop mid-air.
2. You need to be above the world height in order to do this. If you descend below it, it will stop working.
3. It does use elytra durability.
4. This also works with upwards bubble columns, but not with lava, slime blocks, jumping normally, taking knockback damage, or any other ways of gaining velocity that we have tested.
5. Thank you to Saltswimmer, BOAAY, and Jetstreamjoejoe for helping test things!