While going through my activity feed, I found MC-147790, and decided to test out in the new 1.14.2-pre4 snapshot.
It was semi-patched, instead of allowing you to fly however, it gave me a strange UI glitch.
I am unsure of whether or not this is only on my end. If anyone else can confirm it happens on their screen that'd be great.
EDIT: Unrelated to MC-147790, instead related to MC-109246. I don't believe it was ever fixed (I could be wrong though.)

How to reproduce:

1. Hold down your Inventory keybind.

Video attached.

Seizure warning just in case.