fix: avoid widening node columns for edge labels

Edge label placement widened the grid column at the midpoint of the
chosen path segment to fit the label text. When that midpoint fell on
a column owned by a node's border, the column grew but the box's own
width did not — leaving a gap between the box and any incoming
arrowhead.

Filter label-line candidates so the widened column is always a free
edge corridor, falling back to a node column only if the path offers
no other segment.

Fixes #63

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: schickling-assistant

cmd/mapping_edge.go

@@ -112,39 +112,74 @@ func (g *graph) determineLabelLine(e *edge) {
 	if lenLabel == 0 {
 		return
 	}
+	// Widening a column that is occupied by a node would push that node's
+	// border out, leaving a visible gap between the box and any incoming
+	// arrowhead. Prefer label-line candidates whose target column is a free
+	// edge corridor; only fall back to a node column if no corridor segment
+	// is available.
 	prevStep := e.path[0]
+	var largestLine []gridCoord
 	var largestLineSize int
-	// Init to first line if we find nothing else
-	largestLine := []gridCoord{prevStep, e.path[1]}
-	largestLineSize = 0
+	var fallbackLine []gridCoord
+	var fallbackLineSize int
 	for _, step := range e.path[1:] {
 		line := []gridCoord{gridCoord(prevStep), gridCoord(step)}
+		prevStep = step
 		lineWidth := g.calculateLineWidth(line)
+		if g.isNodeColumn(labelMiddleX(line)) {
+			if lineWidth > fallbackLineSize {
+				fallbackLineSize = lineWidth
+				fallbackLine = line
+			}
+			continue
+		}
 		if lineWidth >= lenLabel {
 			largestLine = line
 			break
-		} else if lineWidth > largestLineSize {
+		}
+		if lineWidth > largestLineSize {
 			largestLineSize = lineWidth
 			largestLine = line
 		}
-		prevStep = step
 	}
-
-	var maxX, minX int
-	if largestLine[0].x > largestLine[1].x {
-		maxX = largestLine[0].x
-		minX = largestLine[1].x
-	} else {
-		maxX = largestLine[1].x
-		minX = largestLine[0].x
+	if largestLine == nil {
+		largestLine = fallbackLine
 	}
-	middleX := minX + (maxX-minX)/2
+	if largestLine == nil {
+		// Path only had a single segment that lives on a node column; use it
+		// rather than dropping the label entirely.
+		largestLine = []gridCoord{e.path[0], e.path[1]}
+	}
+
+	middleX := labelMiddleX(largestLine)
 	log.Debugf("Increasing column width for column %v from size %v to %v", middleX, g.columnWidth[middleX], lenLabel+2)
 	g.columnWidth[middleX] = Max(g.columnWidth[middleX], lenLabel+2) // Wrap with dashes + arrowhead
 	log.Debugf("New column sizes: %v", g.columnWidth)
 	e.labelLine = largestLine
 }
 
+func labelMiddleX(line []gridCoord) int {
+	minX, maxX := line[0].x, line[1].x
+	if minX > maxX {
+		minX, maxX = maxX, minX
+	}
+	return minX + (maxX-minX)/2
+}
+
+// isNodeColumn reports whether grid column x is occupied by any node.
+// Widening such a column distorts the box that owns it.
+func (g *graph) isNodeColumn(x int) bool {
+	for _, n := range g.nodes {
+		if n.gridCoord == nil {
+			continue
+		}
+		if x >= n.gridCoord.x && x <= n.gridCoord.x+2 {
+			return true
+		}
+	}
+	return false
+}
+
 func (g graph) calculateLineWidth(line []gridCoord) int {
 	totalSize := 0
 	for _, c := range line {

cmd/testdata/ascii/back_edges_two_labels_td.txt

@@ -0,0 +1,31 @@
+graph TD
+A --> B
+B --> C
+C -->|back1| A
+C -->|back2| B
+---
++---+       
+|   |       
+| A |<--+   
+|   |   |   
++---+   |   
+  |     |   
+  |     |   
+  |     |   
+  |     |   
+  v     |   
++---+   |   
+|   |   |   
+| B |<back1 
+|   |   |   
++---+   |   
+  |     |   
+  |     |   
+  |   back2 
+  |     |   
+  v     |   
++---+   |   
+|   |   |   
+| C |---+   
+|   |       
++---+       

cmd/testdata/ascii/bidirectional_edge_labels_lr.txt

@@ -2,10 +2,10 @@ graph LR
 A -->|workload exits| B
 B -->|run| A
 ---
-+---+                +-----+
-|   |                |     |
-| A |workload-exits->|  B  |
-|   |                |     |
-+---+                +-----+
-  ^                    run  
-  +---------------------+   
++---+                +---+
+|   |                |   |
+| A |workload-exits->| B |
+|   |                |   |
++---+                +---+
+  ^                    |  
+  +--------run---------+  

cmd/testdata/ascii/duplicate_edge_labels.txt

@@ -2,10 +2,10 @@ graph LR
 A -->|miss| B
 A -->|hit| B
 ---
-+-----+      +---+
-|     |      |   |
-|  A  |miss->| B |
-|     |      |   |
-+-----+      +---+
-  hit          ^  
-   +-----------+  
++---+      +---+
+|   |      |   |
+| A |miss->| B |
+|   |      |   |
++---+      +---+
+  |          ^  
+  +---hit----+  

cmd/testdata/extended-chars/back_edges_two_labels_td.txt

@@ -0,0 +1,31 @@
+graph TD
+A --> B
+B --> C
+C -->|back1| A
+C -->|back2| B
+---
+┌───┐       
+│   │       
+│ A │◄──┐   
+│   │   │   
+└─┬─┘   │   
+  │     │   
+  │     │   
+  │     │   
+  │     │   
+  ▼     │   
+┌───┐   │   
+│   │   │   
+│ B │◄back1 
+│   │   │   
+└─┬─┘   │   
+  │     │   
+  │     │   
+  │   back2 
+  │     │   
+  ▼     │   
+┌───┐   │   
+│   │   │   
+│ C ├───┘   
+│   │       
+└───┘