dagre: stop container edges at container label

ci/release/changelogs/next.md

@@ -11,6 +11,7 @@
 - CLI reports when a feature is incompatible with layout engine, instead of silently ignoring. [#845](https://github.com/terrastruct/d2/pull/845)
 - `near` key set to direct parent or ancestor throws an appropriate error message. [#851](https://github.com/terrastruct/d2/pull/851)
 - Dimensions and positions are able to be set from API. [#853](https://github.com/terrastruct/d2/pull/853)
+- Improves label legibility for dagre containers by stopping container edges early if they would run into the label. [#880](https://github.com/terrastruct/d2/pull/880)
 
 #### Bugfixes ⛑️
 

d2layouts/d2dagrelayout/layout.go

@@ -425,7 +425,27 @@ func Layout(ctx context.Context, g *d2graph.Graph, opts *ConfigurableOpts) (err
 
 		// trace the edge to the specific shape's border
 		points[startIndex] = shape.TraceToShapeBorder(srcShape, start, points[startIndex+1])
-		points[endIndex] = shape.TraceToShapeBorder(dstShape, end, points[endIndex-1])
+
+		// if an edge to a container runs into its label, stop the edge at the label instead
+		overlapsContainerLabel := false
+		if edge.Dst.IsContainer() && edge.Dst.Attributes.Label.Value != "" {
+			// assumes LabelPosition, LabelWidth, LabelHeight are all set if there is a label
+			labelWidth := float64(*edge.Dst.LabelWidth)
+			labelHeight := float64(*edge.Dst.LabelHeight)
+			labelTL := label.Position(*edge.Dst.LabelPosition).
+				GetPointOnBox(edge.Dst.Box, label.PADDING, labelWidth, labelHeight)
+
+			endingSegment := geo.Segment{Start: points[endIndex-1], End: points[endIndex]}
+			labelBox := geo.NewBox(labelTL, labelWidth, labelHeight)
+			if intersections := labelBox.Intersections(endingSegment); len(intersections) > 0 {
+				overlapsContainerLabel = true
+				// move ending segment to label intersection point
+				points[endIndex] = intersections[0]
+			}
+		}
+		if !overlapsContainerLabel {
+			points[endIndex] = shape.TraceToShapeBorder(dstShape, end, points[endIndex-1])
+		}
 		points = points[startIndex : endIndex+1]
 
 		// build a curved path from the dagre route