fix: detect collisions for nearly axis-aligned segments (closes #1099)

[tcsenpai]

Closes #1099

Problem

segmentIntersectsRect classifies a segment as vertical or horizontal using EPS = 1e-9, then returns false for anything that matches neither. Pin coordinates are not always exactly on the routing grid, so a segment can be skewed by ~6e-5: far below the schematic grid, but five orders of magnitude above 1e-9. Those segments matched neither fast path and were reported as colliding with nothing.

UnroutedTraceRecoverySolver accepts the first candidate that passes pathCollidesWithObstacles, so a skewed candidate was accepted and emitted as the final trace, running straight through a chip body it does not terminate on.

Fix

When a segment matches neither fast path, classify it by its dominant axis if the off-axis deviation is under 1e-3 and test it as the axis-aligned segment it was meant to be. Genuinely diagonal segments still return false, so the change is limited to sub-grid skew.

The 1e-3 threshold sits below the schematic grid, so anything under it is float noise or an off-grid pin, never a deliberate diagonal.

Tests

tests/repros/near-axis-aligned-trace-through-chip.test.ts covers both levels:

• the helper directly, asserting a skewed segment collides with a rect it passes through while a true diagonal still does not
• the pipeline end to end, asserting no trace segment crosses the interior of a chip it does not terminate on

Both assertions fail on main and pass with this change.

bun test: 343 pass, 4 skip, 0 fail (main is also 0 fail, so no regressions).
bunx tsc --noEmit: clean.
No snapshots changed.

Provenance

Found by differential fuzzing over the fixtures in tests/bug-reports and tests/examples, judging each mutated variant against its base fixture's baseline. Two independent fuzzed inputs reduce to this same root cause. The reproducer in the test is the delta-debugged minimum: 3 chips, 1 connection.

This change was prepared with AI assistance and reviewed by the author.

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Project	Deployment	Actions	Updated
schematic-trace-solver	Ready	Preview	Sep 8, 2026 1:09pm UTC