Fix multibyte UTF-8 (Cyrillic) rendering in drawText

Make drawText rune-aware: size using runewidth.StringWidth and iterate
over runes instead of bytes so multibyte characters render in a single
cell instead of being split into invalid byte fragments. Add a
regression test for Cyrillic node and edge labels.

Co-authored-by: Junie <junie@jetbrains.com>

Author: igor-chernikov

cmd/draw.go

@@ -48,11 +48,21 @@ func (g *graph) drawEdge(e *edge) (*drawing, *drawing, *drawing, *drawing, *draw
 }
 
 func (d *drawing) drawText(start drawingCoord, text string) {
-	// Increase dimensions if necessary.
-	d.increaseSize(start.x+len(text), start.y)
-	log.Debug("Drawing '", text, "' from ", start, " to ", drawingCoord{x: start.x + len(text), y: start.y})
-	for x := 0; x < len(text); x++ {
-		(*d)[x+start.x][start.y] = string(text[x])
+	// Increase dimensions if necessary. Use the visual width so multibyte
+	// (e.g. Cyrillic, CJK) runes reserve the correct number of cells.
+	textWidth := runewidth.StringWidth(text)
+	d.increaseSize(start.x+textWidth, start.y)
+	log.Debug("Drawing '", text, "' from ", start, " to ", drawingCoord{x: start.x + textWidth, y: start.y})
+	// Iterate over runes (not bytes) so multibyte characters are placed in a
+	// single cell instead of being split into invalid byte fragments.
+	textX := start.x
+	for _, r := range text {
+		runeWidth := Max(runewidth.RuneWidth(r), 1)
+		(*d)[textX][start.y] = string(r)
+		for offset := 1; offset < runeWidth; offset++ {
+			(*d)[textX+offset][start.y] = ""
+		}
+		textX += runeWidth
 	}
 }
 

cmd/render_graph_test.go

@@ -146,6 +146,28 @@ func TestRenderGraphSeparatesBidirectionalEdgeLabelsTD(t *testing.T) {
 	}
 }
 
+func TestRenderGraphRendersCyrillicNodeAndEdgeLabels(t *testing.T) {
+	config := diagram.NewTestConfig(true, "cli")
+	output, err := RenderDiagram("graph LR\nA[Привет]-->|метка|B[Мир]", config)
+	if err != nil {
+		t.Fatalf("RenderDiagram() error = %v", err)
+	}
+
+	for _, want := range []string{"Привет", "метка", "Мир"} {
+		if !strings.Contains(output, want) {
+			t.Fatalf("expected output to contain Cyrillic label %q\noutput:\n%s", want, output)
+		}
+	}
+
+	// The replacement character indicates a multibyte rune was split into
+	// invalid byte fragments during rendering.
+	if strings.ContainsRune(output, '\uFFFD') {
+		t.Fatalf("expected no replacement characters in Cyrillic output\noutput:\n%s", output)
+	}
+
+	assertUniformDisplayWidth(t, output)
+}
+
 func assertUniformDisplayWidth(t *testing.T, output string) {
 	t.Helper()