Container: order floating children by depth alone, not by screen position

[obiot]

The bug

Container.draw gives a floating child resetTransform() and the camera's screen projection, so its pos.x/y are canvas pixels — not a place in the world. _sortDepth fed those to a world-space distance and subtracted the camera position on top of that.

Two consequences, both under a Camera3d:

A HUD's layering depended on where it sat on the screen.

	screen pos	contributed to the distance	result
score, in a corner	(20, 16)	656	sorted near → drew on top
banner, centred	(512, 200)	302144	sorted far → scenery drew over it

Same floating, same depth. Only the screen position differed.

And it drifted as the camera travelled, via the (z - camZ)² term — a HUD correct at the start of a level was buried by the end of it, which is the kind of thing that gets papered over with a per-frame hud.depth = camera.pos.z + n workaround.

The fix

A floating child is ordered by |pos.z| alone. World children are untouched.

Magnitude is what the screen-space idioms in this repo already encode, and the old key squared the distance, so the sign never carried meaning:

world.addChild(hud, -150);        // small -> near -> in front of the scene
world.addChild(backdrop, -10000); // large -> far  -> behind the scene
world.addChild(backdrop, 100000); // equally far; the sign is not read

That is the flight demo's HUD at -150, and the glTF Scene, glTF Animated Model, Billboard and Night City examples parking a floating sky at -10000, and Instanced Forest at 100000. Treating a negative depth as "nearest" pulls those four skies in front of their own scenes and leaves nothing but a gradient — so the sign is ignored.

Tests

tests/floating-depth-sort.spec.js, 21 tests: both idioms at extreme camera positions, a backdrop with the camera sitting on its own depth, an overlay after 8000 units of travel, sign symmetry, and the world path asserted unchanged.

Checked against four mutations, each of which is a fix someone could plausibly write:

mutation	tests failing
the pre-fix key	10 / 21
floating forced to the front	11 / 21
a signed key	5 / 21
keeping the screen coordinates, dropping only the camera term	6 / 21

Verification

• 267 files / 6480 tests pass; lint, types and biome clean
• every 3D example opened and inspected: glTF Scene, glTF Animated Model, Billboard, Night City, Instanced Forest, AfterBurner, Per-material Textures
• 2D examples are unaffected — their worlds sort on "z", not "depth"

Supersedes #1628, which forced floating children to the front and blanked three examples.

🤖 Generated with Claude Code

https://claude.ai/code/session_012Aa37KGXZcnVrbn1yG4j1N

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