Fix panic in LSP formatting with multi-byte characters and trailing newlines

internal/ls/converters.go

@@ -228,12 +228,17 @@ func (c *Converters) PositionToLineAndCharacter(script Script, position core.Tex
 
 	start := lineMap.LineStarts[line]
 
+	// Ensure position doesn't exceed text length to avoid slice bounds errors
+	text := script.Text()
+	textLen := core.TextPos(len(text))
+	position = min(position, textLen)
+
 	var character core.TextPos
 	if lineMap.AsciiOnly || c.positionEncoding == lsproto.PositionEncodingKindUTF8 {
 		character = position - start
 	} else {
 		// We need to rescan the text as UTF-16 to find the character offset.
-		for _, r := range script.Text()[start:position] {
+		for _, r := range text[start:position] {
 			character += core.TextPos(utf16.RuneLen(r))
 		}
 	}

internal/ls/converters_bounds_test.go

@@ -0,0 +1,56 @@
+package ls
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/core"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+)
+
+type mockScript struct {
+	fileName string
+	text     string
+}
+
+func (m *mockScript) FileName() string {
+	return m.fileName
+}
+
+func (m *mockScript) Text() string {
+	return m.text
+}
+
+func TestPositionToLineAndCharacterBoundsCheck(t *testing.T) {
+	t.Parallel()
+
+	// Test case that reproduces the panic with multi-byte characters and trailing newlines
+	text := "\"→\" ;\n\n\n"
+
+	lineMap := ComputeLineStarts(text)
+
+	converters := NewConverters(lsproto.PositionEncodingKindUTF16, func(fileName string) *LineMap {
+		return lineMap
+	})
+
+	script := &mockScript{
+		fileName: "test.ts",
+		text:     text,
+	}
+
+	// This should not panic even if position is beyond text length
+	textLen := len(text)
+	position := core.TextPos(textLen) // position at end of text
+
+	// This should work
+	result := converters.PositionToLineAndCharacter(script, position)
+
+	t.Logf("Text length: %d, Position: %d", textLen, position)
+	t.Logf("Result: line=%d, char=%d", result.Line, result.Character)
+
+	// This should also not panic, even though position is beyond text length
+	beyondEndPosition := core.TextPos(textLen + 1)
+	result2 := converters.PositionToLineAndCharacter(script, beyondEndPosition)
+
+	t.Logf("Beyond end position: %d", beyondEndPosition)
+	t.Logf("Result2: line=%d, char=%d", result2.Line, result2.Character)
+}