fix: support CJK/Unicode character rendering in sequence diagrams

Use a custom runewidth.Condition with EastAsianWidth=false to properly
calculate display widths for both box drawing characters and CJK text.

The issue: runewidth library treats box drawing characters (┌, ─, │) as
East Asian Ambiguous, returning width=2, while they actually display as
width=1 in most terminals. This caused misalignment when CJK characters
(which correctly have width=2) were used in participant names.

Changes:
- Add widthCondition with EastAsianWidth=false for consistent width calculation
- Replace runewidth.StringWidth() with widthCondition.StringWidth()
- Rewrite label rendering to use display width-based positioning
- Add east_asian_characters.txt test case with Japanese, Korean, Chinese

Based on upstream PR AlexanderGrooff#55 by ColtWindy, adapted for multi-line label support.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: drunkhacker

cmd/testdata/sequence/east_asian_characters.txt

@@ -0,0 +1,25 @@
+sequenceDiagram
+    participant C as Client
+    participant S as サーバー
+    participant DB as 데이터베이스
+    C->>S: Login Request
+    S->>DB: ユーザー検索
+    DB-->>S: 사용자 정보
+    S-->>C: Success 成功
+---
+┌────────┐     ┌──────────┐     ┌──────────────┐
+│ Client │     │ サーバー │     │ 데이터베이스 │
+└────┬───┘     └─────┬────┘     └───────┬──────┘
+     │               │                  │
+     │ Login Request │                  │
+     ├──────────────►│                  │
+     │               │                  │
+     │               │ ユーザー検索     │
+     │               ├─────────────────►│
+     │               │                  │
+     │               │ 사용자 정보      │
+     │               │◄┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┤
+     │               │                  │
+     │ Success 成功  │                  │
+     │◄┈┈┈┈┈┈┈┈┈┈┈┈┈┈┤                  │
+     │               │                  │

internal/sequence/renderer.go

@@ -19,6 +19,11 @@ const (
 	labelBufferSpace          = 10
 )
 
+// widthCondition is used for calculating display width.
+// EastAsianWidth is set to false so that box drawing characters
+// are treated as narrow (width=1) while CJK characters remain wide (width=2).
+var widthCondition = &runewidth.Condition{EastAsianWidth: false}
+
 type diagramLayout struct {
 	participantWidths  []int
 	participantCenters []int
@@ -35,7 +40,7 @@ func calculateLayout(sd *SequenceDiagram, config *diagram.Config) *diagramLayout
 
 	widths := make([]int, len(sd.Participants))
 	for i, p := range sd.Participants {
-		w := runewidth.StringWidth(p.Label) + boxPaddingLeftRight
+		w := widthCondition.StringWidth(p.Label) + boxPaddingLeftRight
 		if w < minBoxWidth {
 			w = minBoxWidth
 		}
@@ -99,7 +104,7 @@ func Render(sd *SequenceDiagram, config *diagram.Config) (string, error) {
 
 	lines = append(lines, buildLine(sd.Participants, layout, func(i int) string {
 		w := layout.participantWidths[i]
-		labelLen := runewidth.StringWidth(sd.Participants[i].Label)
+		labelLen := widthCondition.StringWidth(sd.Participants[i].Label)
 		pad := (w - labelLen) / 2
 		return string(chars.Vertical) + strings.Repeat(" ", pad) + sd.Participants[i].Label +
 			strings.Repeat(" ", w-pad-labelLen) + string(chars.Vertical)
@@ -134,7 +139,7 @@ func buildLine(participants []*Participant, layout *diagramLayout, draw func(int
 		boxWidth := layout.participantWidths[i] + boxBorderWidth
 		left := layout.participantCenters[i] - boxWidth/2
 
-		needed := left - len([]rune(sb.String()))
+		needed := left - widthCondition.StringWidth(sb.String())
 		if needed > 0 {
 			sb.WriteString(strings.Repeat(" ", needed))
 		}
@@ -169,25 +174,42 @@ func renderMessage(msg *Message, layout *diagramLayout, chars BoxChars) []string
 		start := min(from, to) + labelLeftMargin
 		labelLines := strings.Split(label, "\n")
 		for _, labelLine := range labelLines {
-			labelWidth := runewidth.StringWidth(labelLine)
-			w := max(layout.totalWidth, start+labelWidth) + labelBufferSpace
-			line := []rune(buildLifeline(layout, chars))
-			if len(line) < w {
-				padding := make([]rune, w-len(line))
-				for k := range padding {
-					padding[k] = ' '
+			labelWidth := widthCondition.StringWidth(labelLine)
+			totalW := max(layout.totalWidth, start+labelWidth) + labelBufferSpace
+
+			// Build the line using display width positioning
+			var sb strings.Builder
+			pos := 0 // current display position
+			labelRunes := []rune(labelLine)
+			labelIdx := 0
+
+			for pos < totalW {
+				// Label takes priority over lifelines (overwrites them)
+				if pos >= start && labelIdx < len(labelRunes) {
+					r := labelRunes[labelIdx]
+					sb.WriteRune(r)
+					pos += widthCondition.RuneWidth(r)
+					labelIdx++
+				} else {
+					// Check if current position is a lifeline
+					isLifeline := false
+					for _, c := range layout.participantCenters {
+						if pos == c {
+							isLifeline = true
+							break
+						}
+					}
+
+					if isLifeline {
+						sb.WriteRune(chars.Vertical)
+					} else {
+						sb.WriteRune(' ')
+					}
+					pos++
 				}
-				line = append(line, padding...)
 			}
 
-			col := start
-			for _, r := range labelLine {
-				if col < len(line) {
-					line[col] = r
-					col++
-				}
-			}
-			lines = append(lines, strings.TrimRight(string(line), " "))
+			lines = append(lines, strings.TrimRight(sb.String(), " "))
 		}
 	}
 
@@ -243,24 +265,42 @@ func renderSelfMessage(msg *Message, layout *diagramLayout, chars BoxChars) []st
 		start := center + labelLeftMargin
 		labelLines := strings.Split(label, "\n")
 		for _, labelLine := range labelLines {
-			line := ensureWidth(buildLifeline(layout, chars))
-			labelWidth := runewidth.StringWidth(labelLine)
-			needed := start + labelWidth + labelBufferSpace
-			if len(line) < needed {
-				pad := make([]rune, needed-len(line))
-				for i := range pad {
-					pad[i] = ' '
+			labelWidth := widthCondition.StringWidth(labelLine)
+			totalW := start + labelWidth + labelBufferSpace
+
+			// Build the line using display width positioning
+			var sb strings.Builder
+			pos := 0 // current display position
+			labelRunes := []rune(labelLine)
+			labelIdx := 0
+
+			for pos < totalW {
+				// Label takes priority over lifelines
+				if pos >= start && labelIdx < len(labelRunes) {
+					r := labelRunes[labelIdx]
+					sb.WriteRune(r)
+					pos += widthCondition.RuneWidth(r)
+					labelIdx++
+				} else {
+					// Check if current position is a lifeline
+					isLifeline := false
+					for _, c := range layout.participantCenters {
+						if pos == c {
+							isLifeline = true
+							break
+						}
+					}
+
+					if isLifeline {
+						sb.WriteRune(chars.Vertical)
+					} else {
+						sb.WriteRune(' ')
+					}
+					pos++
 				}
-				line = append(line, pad...)
 			}
-			col := start
-			for _, c := range labelLine {
-				if col < len(line) {
-					line[col] = c
-					col++
-				}
-			}
-			lines = append(lines, strings.TrimRight(string(line), " "))
+
+			lines = append(lines, strings.TrimRight(sb.String(), " "))
 		}
 	}
 

internal/sequence/renderer_test.go

@@ -29,6 +29,7 @@ func TestSequenceDiagramRendering(t *testing.T) {
 		"autonumber.txt",
 		"bidirectional_messages.txt",
 		"dotted_arrows_only.txt",
+		"east_asian_characters.txt",
 		"four_participants.txt",
 		"long_participant_names.txt",
 		"messages_without_labels.txt",
@@ -76,6 +77,7 @@ func TestSequenceDiagramRendering_ASCIISmokeTest(t *testing.T) {
 		"autonumber.txt",
 		"bidirectional_messages.txt",
 		"dotted_arrows_only.txt",
+		"east_asian_characters.txt",
 		"four_participants.txt",
 		"long_participant_names.txt",
 		"messages_without_labels.txt",