player/player.go: Stop treating blocks passed while falling as ground

[root-nat]

Reworked from #1283 per @TwistedAsylumMC's review: this fixes the root cause in checkOnGround rather than patching the fall-damage path. (The original PR could not be reopened because the branch was force-pushed while it was closed.)

Problem

When a player falls close to a block at its side it takes premature fall damage (#1003). The cause is checkOnGround: it extended the probe box opposite to the whole movement (deltaPos.Mul(-1.0)), which while falling pushes the box upwards and sideways, so a block next to the fall path intersects it and the player is wrongly reported on the ground.

Fix

Extend the probe straight down only — by 0.05 plus the tick's descent — keeping the player's exact horizontal footprint:

.Extend(mgl64.Vec3{0, -0.05}).Extend(mgl64.Vec3{0, math.Min(deltaPos[1], 0)})

A block at the side is no longer mistaken for ground, while a stepped or fast descent still finds the ground beneath the footprint. updateFallState is left untouched.

Testing

Verified in-game on 1.26.30:

• sprinting and walking down a 15-step staircase deals no fall damage (Fix fall damage when sprinting down stairs #1044 preserved);
• a ~15-block open fall deals the expected ~6 hearts;
• falling flush alongside a wall deals the same damage, and only on landing (Fall damage when getting close to blocks while falling #1003 fixed).

Fixes #1003.

🤖 Generated with Claude Code

[FDUTCH]

missunderstanding the cause... (won't work) 👏

[root-nat]

Thanks for taking a look — could you share a concrete failing scenario? I'd like to make sure I'm not missing something.

The change keeps the player's exact X/Z footprint and extends the probe box straight down by 0.05 plus this tick's descent (math.Min(deltaPos[1], 0)), instead of the old .Extend(deltaPos.Mul(-1.0)). While falling, deltaPos[1] is negative, so the old -deltaPos extended the box upward and sideways (opposite the movement) — that is what made it intersect a wall the player was sliding past and report onGround every tick, continuously resetting fallDistance (the #1003 bug). The new box only grows downward under the feet, so a block beside the fall path is no longer treated as ground while ground actually under the feet still is.

I tested this in-game on 1.26.30: sprinting/walking down stairs deals no fall damage (#1044 stays fixed), a ~15-block open fall deals ~6 hearts, and falling flush against a wall now deals damage only on landing (#1003 fixed). I also checked the edge cases (standing, ledges, jumping where math.Min clamps to 0, and fast falls where checkBlockCollisions has already clamped the position onto the floor).

If your point is that onGround should ideally come from the move's vertical-collision result (like vanilla) rather than a probe box — I agree in principle, but checkBlockCollisions only runs when deltaPos.Len() <= 3 and collidedVertically is also set on ceiling hits, so switching to it is a riskier refactor I'd rather keep separate from this fix. Happy to dig into any specific repro you have.

[SpaceLostC]

Absolutely! Here's an example scenario of this happening: https://www.youtube.com/watch?v=IlZ-Lclu54E