feat: handle insert mode

handler.go

@@ -391,6 +391,8 @@ func (v *Terminal) SetMode(mode ansicode.TerminalMode) {
 	switch mode {
 	case ansicode.TerminalModeCursorKeys:
 		forward = true
+	case ansicode.TerminalModeInsert:
+		v.insertMode = true
 	case ansicode.TerminalModeLineWrap:
 	case ansicode.TerminalModeBlinkingCursor:
 		epoch := time.Now()
@@ -627,6 +629,8 @@ func (v *Terminal) UnsetMode(mode ansicode.TerminalMode) {
 	switch mode {
 	case ansicode.TerminalModeCursorKeys:
 		forward = true
+	case ansicode.TerminalModeInsert:
+		v.insertMode = false
 	case ansicode.TerminalModeLineWrap:
 	case ansicode.TerminalModeBlinkingCursor:
 		v.CursorBlinkEpoch = nil

terminal.go

@@ -52,6 +52,10 @@ type Terminal struct {
 	// to the next line if another character is printed.
 	wrap bool
 
+	// insertMode indicates whether printable input
+	// should shift row contents right.
+	insertMode bool
+
 	*ansicode.Decoder
 
 	// onResize is a hook called every time the terminal resizes.
@@ -147,6 +151,7 @@ func (v *Terminal) Reset() {
 	v.mut.Lock()
 	defer v.mut.Unlock()
 	v.reset()
+	v.insertMode = false
 }
 
 func (v *Terminal) UsedHeight() int {
@@ -247,6 +252,9 @@ func (v *Terminal) put(r rune) {
 		v.wrap = false
 	}
 	x, y, f := v.Cursor.X, v.Cursor.Y, v.Cursor.F
+	if v.insertMode {
+		v.insertCharacters(1)
+	}
 	v.paint(y, x, f, r)
 	if y > v.MaxY {
 		v.MaxY = y

terminal_test.go

@@ -202,8 +202,70 @@ func TestResizeGrowingHeightThenShrinkWidth(t *testing.T) {
 	require.NoError(t, err)
 }
 
-func eachFrame(r io.Reader, callback func(frame int, segment []byte)) {
-	eachNthFrame(r, 1, callback)
+func TestInsertModePreservesShiftedContentAcrossLines(t *testing.T) {
+	term := midterm.NewTerminal(24, 80)
+	term.Raw = true
+
+	// Seed the two-line prompt before the redraw:
+	//
+	// Please answer: first
+	// (▼ for other options)
+	_, err := io.WriteString(term, "\r\x1b[JPlease answer: first \r\n(▼ for other options)")
+	require.NoError(t, err)
+
+	// Replace "first" with "second",
+	// and insert "▲" and "▼" on the next line
+	// so the existing space shifts:
+	//
+	// Please answer: second
+	// (▲▼ for other options)
+	_, err = io.WriteString(term, "\x1b[A\x1b[6Dsecond\x1b[4h \x1b[4l\r\n\x1b[C▲\x1b[4h▼\x1b[4l")
+	require.NoError(t, err)
+
+	require.Equal(t, "Please answer: second", strings.TrimRight(string(term.Content[0]), " "))
+	require.Equal(t, "(▲▼ for other options)", strings.TrimRight(string(term.Content[1]), " "))
+}
+
+func TestInsertModeShiftsSingleLineContent(t *testing.T) {
+	term := midterm.NewTerminal(1, 8)
+	term.Raw = true
+
+	// Start with the cursor after the final "e":
+	//
+	// abcde^
+	_, err := io.WriteString(term, "abcde")
+	require.NoError(t, err)
+
+	// Return to column 3, enable insert mode, and insert a space:
+	//
+	// abc^de
+	// abc ^de
+	_, err = io.WriteString(term, "\r\x1b[3C\x1b[4h \x1b[4l")
+	require.NoError(t, err)
+
+	require.Equal(t, "abc de", strings.TrimRight(string(term.Content[0]), " "))
+}
+
+func TestUnsetInsertModeRestoresReplaceMode(t *testing.T) {
+	term := midterm.NewTerminal(1, 8)
+	term.Raw = true
+
+	// Start with the cursor after the final "e":
+	//
+	// abcde^
+	_, err := io.WriteString(term, "abcde")
+	require.NoError(t, err)
+
+	// Insert a space at column 3, disable insert mode, then overwrite the "d"
+	// with "Z" at the next cursor position:
+	//
+	// abc^de
+	// abc ^de
+	// abc Z^e
+	_, err = io.WriteString(term, "\r\x1b[3C\x1b[4h \x1b[4lZ")
+	require.NoError(t, err)
+
+	require.Equal(t, "abc Ze", strings.TrimRight(string(term.Content[0]), " "))
 }
 
 func eachNthFrame(r io.Reader, n int, callback func(frame int, segment []byte)) {