ELK non-layered algorithms panic on edges to deeply-nested nodes

[Gerg]

Summary

ELK algorithms other than layered panic with "index out of range" when an edge connects an external node to a node inside a nested container.

Reproduction

d2 --version
0.7.1

test.d2:

outer: {
  inner: {
    a
  }
}
external -> outer.inner.a

Working:

❯ d2 --layout=elk --elk-algorithm=layered test.d2 test.png
success: successfully compiled test.d2 to test.png in 372.446125ms

Error:

d2 --layout=elk --elk-algorithm=stress test.d2 test.png
panic: runtime error: index out of range [0] with length 0

goroutine 8 [running]:
oss.terrastruct.com/d2/d2layouts/d2elklayout.Layout(...)
	oss.terrastruct.com/d2/d2layouts/d2elklayout/layout.go:604 +0x1dcc

Root Cause

d2/d2layouts/d2elklayout/layout.go

Lines 601 to 605 in 93f9720

	points := edge.Route
	startIndex, endIndex := 0, len(points)-1
	start := points[startIndex]
	end := points[endIndex]

Non-layered ELK algorithms return empty Route arrays for edges that cross container boundaries. The code assumes edge.Route is non-empty.

AI Disclaimer

Issue diagnosed with assistance from Claude 4.5 Opus.