gitstatus: clip diff lines by glyph, not by rune

internal/plugins/gitstatus/diff_renderer.go

@@ -7,7 +7,7 @@ import (
 	"charm.land/lipgloss/v2"
 	"github.com/charmbracelet/x/ansi"
 	"github.com/marcus/sidecar/internal/styles"
-	"github.com/mattn/go-runewidth"
+	"github.com/rivo/uniseg"
 )
 
 // DiffViewMode specifies the diff rendering mode.
@@ -903,12 +903,23 @@ func clipToWidth(s string, maxWidth int) (string, bool) {
 	if strings.IndexByte(s, 0x1b) >= 0 {
 		return s, lipgloss.Width(s) > maxWidth
 	}
+	// Step a grapheme cluster at a time, measured the way lipgloss.Width above
+	// and padRight below measure. A cluster is what a terminal draws as one
+	// character and may be several runes: a flag, an emoji carrying a skin
+	// tone, a family joined by zero-width joiners. Added up rune by rune a
+	// family counts as eight columns where it occupies two, so the clip and
+	// the pad disagree about the same line, and a cut between its runes
+	// leaves a dangling joiner.
 	width := 0
-	for i, r := range s {
+	rest := s
+	state := -1
+	for len(rest) > 0 {
 		if width > maxWidth {
-			return s[:i], true
+			return s[:len(s)-len(rest)], true
 		}
-		width += runewidth.RuneWidth(r)
+		var cluster string
+		cluster, rest, _, state = uniseg.FirstGraphemeClusterInString(rest, state)
+		width += ansi.StringWidth(cluster)
 	}
 	return s, width > maxWidth
 }

internal/plugins/gitstatus/diff_renderer_test.go

@@ -3,6 +3,8 @@ package gitstatus
 import (
 	"strings"
 	"testing"
+
+	"github.com/rivo/uniseg"
 )
 
 func TestRenderLineDiff_EmptyDiff(t *testing.T) {
@@ -209,6 +211,42 @@ func TestPadRight(t *testing.T) {
 	}
 }
 
+// A terminal draws each of these as one character, out of several runes.
+var multiRuneGlyphs = map[string]string{
+	"family":    "\U0001F468\u200d\U0001F469\u200d\U0001F467\u200d\U0001F466",
+	"rainbow":   "\U0001F3F3\ufe0f\u200d\U0001F308",
+	"skin tone": "\U0001F44D\U0001F3FD",
+	"flag":      "\U0001F1EF\U0001F1F5",
+}
+
+// clipToWidth cuts a line to fit a column. Cutting between the runes of one
+// glyph leaves a dangling zero-width joiner or a lone regional indicator at
+// the end of the line.
+func TestClipToWidth_KeepsGlyphsWhole(t *testing.T) {
+	for name, glyph := range multiRuneGlyphs {
+		t.Run(name, func(t *testing.T) {
+			line := "+ " + strings.Repeat(glyph, 8) + " end"
+
+			boundaries := map[int]bool{0: true}
+			for g := uniseg.NewGraphemes(line); g.Next(); {
+				_, to := g.Positions()
+				boundaries[to] = true
+			}
+
+			for width := 1; width <= 20; width++ {
+				got, _ := clipToWidth(line, width)
+				if !strings.HasPrefix(line, got) {
+					t.Fatalf("clipToWidth(line, %d) is not a prefix of the line", width)
+				}
+				if !boundaries[len(got)] {
+					t.Errorf("clipToWidth(line, %d) cut at byte %d, inside a glyph: %q",
+						width, len(got), got)
+				}
+			}
+		})
+	}
+}
+
 func TestDiffViewMode_Constants(t *testing.T) {
 	// Verify the constants exist and are distinct
 	if DiffViewUnified == DiffViewSideBySide {