This jump's long been in our community known to be impossible but been cleared quite some times without proof anyways.
Soooo ... here the explanation:
1st off: Props to Kinata for finding out almost all of this.
This jump works with numeric instability of pixels. This means: At a certain point in time you y position is a.5. Usually game maker would round it up to a+1. But since Game maker isn't perfect -no shit- it sometimes rounds it down to a instead. This rounding isn't actually random as in RNG but it depends on 3 known factors:
1. Kids y position
2. Kids y position when restarting/spawning
3. Your processor
Depending on your processor there are more or less of those instabilities occuring.
Now to how you do it:
Y=297.305
Perfect bunny hop -5 frame jump when you hit ground hit jump again on 1st or 2nd frame-
19frame jump while holding right
6 frame pause
hold right
as late as you can 5 frame rejump
be happy
Thanks to Kinata again for finding out how this is possible!