Update

internal/mesh/layout.go

@@ -84,16 +84,24 @@ func MergePeers(existing []PeerNode, incoming []viz.Peer) []PeerNode {
 }
 
 func EdgeSpawnPoint(w, h float64, seed int) Point {
+	hSpan := int(h) - 80
+	if hSpan < 1 {
+		hSpan = 1
+	}
+	wSpan := int(w) - 40
+	if wSpan < 1 {
+		wSpan = 1
+	}
 	side := seed % 4
 	switch side {
 	case 0:
-		return Point{X: -20, Y: 40 + float64((seed*17)%(int(h)-80))}
+		return Point{X: -20, Y: 40 + float64((seed*17)%hSpan)}
 	case 1:
-		return Point{X: w + 20, Y: 40 + float64((seed*13)%(int(h)-80))}
+		return Point{X: w + 20, Y: 40 + float64((seed*13)%hSpan)}
 	case 2:
-		return Point{X: 20 + float64((seed*11)%int(w-40)), Y: -20}
+		return Point{X: 20 + float64((seed*11)%wSpan), Y: -20}
 	default:
-		return Point{X: 20 + float64((seed*19)%int(w-40)), Y: h + 20}
+		return Point{X: 20 + float64((seed*19)%wSpan), Y: h + 20}
 	}
 }
 

internal/mesh/layout_test.go

@@ -81,3 +81,16 @@ func TestServicePulseColor(t *testing.T) {
 		t.Fatal("fallback")
 	}
 }
+
+func TestEdgeSpawnPointSmallDimensions(t *testing.T) {
+	// Regression: int(h)-80 or int(w)-40 can be zero and panic on modulo.
+	for _, dims := range [][2]float64{{40, 80}, {16, 16}, {1, 1}, {80, 24}} {
+		w, h := dims[0], dims[1]
+		for seed := 0; seed < 32; seed++ {
+			p := EdgeSpawnPoint(w, h, seed)
+			if math.IsNaN(p.X) || math.IsNaN(p.Y) || math.IsInf(p.X, 0) || math.IsInf(p.Y, 0) {
+				t.Fatalf("bad point for w=%v h=%v seed=%d: %+v", w, h, seed, p)
+			}
+		}
+	}
+}