Fix lastChar()

m/styledStringSplitter.go

@@ -56,6 +56,10 @@ func (s *styledStringSplitter) nextChar() rune {
 
 // Returns whatever the last call to nextChar() returned
 func (s *styledStringSplitter) lastChar() rune {
+	if s.previousByteIndex >= len(s.input) {
+		return -1
+	}
+
 	char, _ := utf8.DecodeRuneInString(s.input[s.previousByteIndex:])
 	return char
 }
@@ -74,8 +78,6 @@ func (s *styledStringSplitter) run() {
 				// Somewhere in handleEscape(), we got a character that was
 				// unexpected. We need to treat everything up to before that
 				// character as just plain runes.
-				//
-				// The character itself, should be retried inside this loop.
 				for _, char := range s.input[escIndex:s.previousByteIndex] {
 					s.handleRune(char)
 				}

m/styledStringSplitter_test.go

@@ -0,0 +1,27 @@
+package m
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestNextCharLastChar_base(t *testing.T) {
+	s := styledStringSplitter{
+		input: "a",
+	}
+
+	assert.Equal(t, 'a', s.nextChar())
+	assert.Equal(t, 'a', s.lastChar())
+	assert.Equal(t, rune(-1), s.nextChar())
+	assert.Equal(t, rune(-1), s.lastChar())
+}
+
+func TestNextCharLastChar_empty(t *testing.T) {
+	s := styledStringSplitter{
+		input: "",
+	}
+
+	assert.Equal(t, rune(-1), s.nextChar())
+	assert.Equal(t, rune(-1), s.lastChar())
+}