fix(whitespace): prevent wide runes from overflowing render width

whitespace.render checked the loop bound only before writing a rune,
then advanced by the rune width. A rune wider than the remaining space
was still written, overshooting the requested width by up to
runeWidth-1 cells, and this made the trailing space-fill unreachable.

Stop before a wide rune would overflow and let the existing space-fill
pad the remaining cells, so the result is always exactly the requested
width. This also fixes overflow surfaced through Place/PlaceHorizontal
with WithWhitespaceChars for multi-cell whitespace characters.

whitespace.go

@@ -33,14 +33,19 @@ func (w whitespace) render(width int) string {
 
 	// Cycle through runes and print them into the whitespace.
 	for i := 0; i < width; {
-		b.WriteRune(r[j])
-		// Measure the width of the rune we just wrote, ensuring we always
-		// make progress to avoid infinite loops with zero-width characters
-		// like tabs.
+		// Measure the width of the current rune, ensuring we always make
+		// progress to avoid infinite loops with zero-width characters like
+		// tabs.
 		runeWidth := ansi.StringWidth(string(r[j]))
 		if runeWidth < 1 {
 			runeWidth = 1
 		}
+		// Stop before a wide rune would overflow the requested width; the
+		// remaining cells are filled with spaces below.
+		if i+runeWidth > width {
+			break
+		}
+		b.WriteRune(r[j])
 		i += runeWidth
 		j++
 		if j >= len(r) {
@@ -48,7 +53,7 @@ func (w whitespace) render(width int) string {
 		}
 	}
 
-	// Fill any extra gaps white spaces. This might be necessary if any runes
+	// Fill any extra gaps with spaces. This might be necessary if any runes
 	// are more than one cell wide, which could leave a one-rune gap.
 	short := width - ansi.StringWidth(b.String())
 	if short > 0 {

whitespace_test.go

@@ -3,6 +3,8 @@ package lipgloss
 import (
 	"testing"
 	"time"
+
+	"github.com/charmbracelet/x/ansi"
 )
 
 func TestWhitespaceRenderWithTab(t *testing.T) {
@@ -50,3 +52,25 @@ func TestWhitespaceRenderNormal(t *testing.T) {
 		t.Errorf("expected 5 characters, got %d", len(result))
 	}
 }
+
+func TestWhitespaceRenderWideRune(t *testing.T) {
+	// A whitespace char that is more than one cell wide must never render
+	// wider than the requested width; the trailing cells are padded with
+	// spaces instead of overflowing.
+	ws := newWhitespace(WithWhitespaceChars("東")) // width 2
+	for _, width := range []int{0, 1, 2, 3, 4, 5} {
+		got := ansi.StringWidth(ws.render(width))
+		if got != width {
+			t.Errorf("render(%d): expected rendered width %d, got %d", width, width, got)
+		}
+	}
+}
+
+func TestPlaceHorizontalWideWhitespace(t *testing.T) {
+	// Placing content in a box with a wide whitespace char must keep the
+	// box exactly the requested width.
+	got := PlaceHorizontal(5, Left, "ab", WithWhitespaceChars("東"))
+	if w := ansi.StringWidth(got); w != 5 {
+		t.Errorf("PlaceHorizontal width: expected 5, got %d (%q)", w, got)
+	}
+}