player/player.go: Only deal fall damage on an actual vertical collision

[root-nat]

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. This is the root-cause fix suggested in the previous review, rather than patching the fall-damage path.

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

[TwistedAsylumMC]

We should fix the on ground being set to true when falling past blocks rather than this

[root-nat]

Thanks for the steer — agreed, patching updateFallState was treating the symptom.

I've reworked it to fix checkOnGround directly. The probe box was extended opposite to the whole movement (deltaPos.Mul(-1.0)), which while falling pushes it upwards and sideways, so a block next to the fall path intersects it and the player reports as grounded. The rework extends 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, so the sprinting-down-stairs fix from #1044 should hold (the 0.6-wide footprint always straddles the step it is leaving, and the descent term keeps the downward reach in step with the gap). The idle-tick call, which passes a zero delta, is unchanged.

Does this match what you had in mind? If so I'll reopen with it — I'll give the stairs/slabs case a run in-game first either way.

[root-nat]

Continued in #1286 with the root-cause checkOnGround rework (this PR could not be reopened after the branch was force-pushed).