Fix --gap-line cutting a grapheme cluster

RepeatToFill filled the remaining width rune by rune, so a cluster
could be split at the right edge. Iterate grapheme clusters instead.

Fix #4920

CHANGELOG.md

@@ -1,6 +1,10 @@
 CHANGELOG
 =========
 
+0.74.5
+------
+- Fixed `--gap-line` cutting a grapheme cluster when filling the last cells of the line (#4920)
+
 0.74.4
 ------
 - Fixed an escape sequence split across reads being parsed as a fragment, which leaked the rest into the query (#4899)

src/util/util.go

@@ -105,16 +105,18 @@ func RepeatToFill(str string, length int, limit int) string {
 	rest := limit % length
 	output := strings.Repeat(str, times)
 	if rest > 0 {
-		for _, r := range str {
-			rest -= uniseg.StringWidth(string(r))
-			if rest < 0 {
-				break
-			}
-			output += string(r)
-			if rest == 0 {
+		// Iterate over grapheme clusters so that we don't cut a cluster in half
+		end := 0
+		graphemes := uniseg.NewGraphemes(str)
+		for rest > 0 && graphemes.Next() {
+			width := graphemes.Width()
+			if width > rest {
 				break
 			}
+			rest -= width
+			_, end = graphemes.Positions()
 		}
+		output += str[:end]
 	}
 	return output
 }

src/util/util_test.go

@@ -109,6 +109,27 @@ func TestRepeatToFill(t *testing.T) {
 	if RepeatToFill("abcde", 10, 42) != strings.Repeat("abcde", 4)+"abcde"[:2] {
 		t.Error("Expected:", strings.Repeat("abcde", 4)+"abcde"[:2])
 	}
+
+	// Should not cut a grapheme cluster in half
+	for _, test := range []struct {
+		str      string
+		limit    int
+		expected string
+	}{
+		{"a\u0301b", 1, "a\u0301"},
+		{"a\u0301b", 3, "a\u0301ba\u0301"},
+		{"a\u0301b", 4, "a\u0301ba\u0301b"},
+		{"a\u4e00", 2, "a"},
+		{"a\u4e00", 4, "a\u4e00a"},
+		{"-\U0001f468\u200d\U0001f469\u200d\U0001f467", 1, "-"},
+		{"-\U0001f468\u200d\U0001f469\u200d\U0001f467", 2, "-"},
+		{"-\U0001f468\u200d\U0001f469\u200d\U0001f467", 4, "-\U0001f468\u200d\U0001f469\u200d\U0001f467-"},
+	} {
+		actual := RepeatToFill(test.str, StringWidth(test.str), test.limit)
+		if actual != test.expected {
+			t.Errorf("Expected: %q, actual: %q", test.expected, actual)
+		}
+	}
 }
 
 func TestStringWidth(t *testing.T) {