terminal: Do not write unchanged status lines

Check if each line of status is changed, and write
the line to the terminal only if it has changed

changelog/unreleased/issue-5562

@@ -0,0 +1,10 @@
+Enhancement: Do not rewrite unchanged lines every frame in status bar
+
+Status bars were entirely rewritten every frame if any of its content has updated.
+This behavior had made any user interaction (such as selection) with status bar
+impossible in certain terminal emulators, even with the unchanged lines.
+Now it writes changed lines only at status bar update, thereby improving 
+user experience in those terminal emulators.
+
+https://github.com/restic/restic/issues/5562
+https://github.com/restic/restic/pull/5648

internal/terminal/terminal_posix.go

@@ -11,6 +11,8 @@ const (
 	PosixControlMoveCursorHome = "\r"
 	// PosixControlMoveCursorUp moves cursor up one line
 	PosixControlMoveCursorUp = "\x1b[1A"
+	// PosixControlMoveCursorDown moves cursor down one line
+	PosixControlMoveCursorDown = "\x1b[1B"
 	// PosixControlClearLine clears the current line
 	PosixControlClearLine = "\x1b[2K"
 )
@@ -36,3 +38,14 @@ func PosixMoveCursorUp(wr io.Writer, _ uintptr, n int) error {
 	}
 	return nil
 }
+
+// PosixMoveCursorDown moves the cursor to the line n lines below the current one.
+func PosixMoveCursorDown(wr io.Writer, _ uintptr, n int) error {
+	data := []byte(PosixControlMoveCursorHome)
+	data = append(data, bytes.Repeat([]byte(PosixControlMoveCursorDown), n)...)
+	_, err := wr.Write(data)
+	if err != nil {
+		return fmt.Errorf("write failed: %w", err)
+	}
+	return nil
+}

internal/terminal/terminal_unix.go

@@ -20,6 +20,11 @@ func MoveCursorUp(_ uintptr) func(io.Writer, uintptr, int) error {
 	return PosixMoveCursorUp
 }
 
+// MoveCursorDown moves the cursor to the line n lines below the current one.
+func MoveCursorDown(_ uintptr) func(io.Writer, uintptr, int) error {
+	return PosixMoveCursorDown
+}
+
 // CanUpdateStatus returns true if status lines can be printed, the process
 // output is not redirected to a file or pipe.
 func CanUpdateStatus(fd uintptr) bool {

internal/terminal/terminal_windows.go

@@ -35,6 +35,17 @@ func MoveCursorUp(fd uintptr) func(io.Writer, uintptr, int) error {
 	return PosixMoveCursorUp
 }
 
+// moveCursorDown moves the cursor to the line n lines below the current one.
+func MoveCursorDown(fd uintptr) func(io.Writer, uintptr, int) error {
+	// easy case, the terminal is cmd or psh, without redirection
+	if isWindowsTerminal(fd) {
+		return windowsMoveCursorDown
+	}
+
+	// assume we're running in mintty/cygwin
+	return PosixMoveCursorDown
+}
+
 var kernel32 = syscall.NewLazyDLL("kernel32.dll")
 
 var (
@@ -73,6 +84,19 @@ func windowsMoveCursorUp(_ io.Writer, fd uintptr, n int) error {
 	return nil
 }
 
+// windowsMoveCursorDown moves the cursor to the line n lines below the current one.
+func windowsMoveCursorDown(_ io.Writer, fd uintptr, n int) error {
+	var info windows.ConsoleScreenBufferInfo
+	windows.GetConsoleScreenBufferInfo(windows.Handle(fd), &info)
+
+	// move cursor up by n lines and to the first column
+	windows.SetConsoleCursorPosition(windows.Handle(fd), windows.Coord{
+		X: 0,
+		Y: info.CursorPosition.Y + int16(n),
+	})
+	return nil
+}
+
 // isWindowsTerminal return true if the file descriptor is a windows terminal (cmd, psh).
 func isWindowsTerminal(fd uintptr) bool {
 	return term.IsTerminal(int(fd))

internal/ui/termstatus/status.go

@@ -40,6 +40,7 @@ type Terminal struct {
 
 	clearCurrentLine func(io.Writer, uintptr) error
 	moveCursorUp     func(io.Writer, uintptr, int) error
+	moveCursorDown   func(io.Writer, uintptr, int) error
 }
 
 type message struct {
@@ -123,6 +124,7 @@ func New(rd io.ReadCloser, wr io.Writer, errWriter io.Writer, disableStatus bool
 			t.fd = d.Fd()
 			t.clearCurrentLine = terminal.ClearCurrentLine(t.fd)
 			t.moveCursorUp = terminal.MoveCursorUp(t.fd)
+			t.moveCursorDown = terminal.MoveCursorDown(t.fd)
 		}
 		if terminal.OutputIsTerminal(d.Fd()) {
 			t.outputIsTerminal = true
@@ -202,6 +204,18 @@ func (t *Terminal) Run(ctx context.Context) {
 	t.runWithoutStatus(ctx)
 }
 
+func findUnchangedLines(curr, last []string) []bool {
+	unchanged := make([]bool, len(curr))
+
+	for i := range min(len(curr), len(last)) {
+		if curr[i] == last[i] {
+			unchanged[i] = true
+		}
+	}
+
+	return unchanged
+}
+
 // run listens on the channels and updates the terminal screen.
 func (t *Terminal) run(ctx context.Context) {
 	var status []string
@@ -210,7 +224,7 @@ func (t *Terminal) run(ctx context.Context) {
 		select {
 		case <-ctx.Done():
 			if !terminal.IsProcessBackground(t.fd) {
-				t.writeStatus([]string{})
+				t.writeStatus([]string{}, nil)
 			}
 
 			return
@@ -241,7 +255,7 @@ func (t *Terminal) run(ctx context.Context) {
 				continue
 			}
 
-			t.writeStatus(status)
+			t.writeStatus(status, nil)
 			lastWrittenStatus = append([]string{}, status...)
 		case stat := <-t.status:
 			status = append(status[:0], stat.lines...)
@@ -252,7 +266,8 @@ func (t *Terminal) run(ctx context.Context) {
 			}
 
 			if !slices.Equal(status, lastWrittenStatus) {
-				t.writeStatus(status)
+				unchangedLines := findUnchangedLines(status, lastWrittenStatus)
+				t.writeStatus(status, unchangedLines)
 				// Copy the status slice to avoid aliasing
 				lastWrittenStatus = append([]string{}, status...)
 			}
@@ -260,7 +275,7 @@ func (t *Terminal) run(ctx context.Context) {
 	}
 }
 
-func (t *Terminal) writeStatus(status []string) {
+func (t *Terminal) writeStatus(status []string, unchanged []bool) {
 	statusLen := len(status)
 	status = append([]string{}, status...)
 	for i := len(status); i < t.lastStatusLen; i++ {
@@ -273,7 +288,18 @@ func (t *Terminal) writeStatus(status []string) {
 	}
 	t.lastStatusLen = statusLen
 
-	for _, line := range status {
+	for i, line := range status {
+		if unchanged != nil && i < len(unchanged) && unchanged[i] {
+			// don't write unchanged lines every frame
+			if i < len(status)-1 {
+				// just move the cursor down to the next line
+				if err := t.moveCursorDown(t.wr, t.fd, 1); err != nil {
+					_, _ = fmt.Fprintf(t.errWriter, "write failed: %v\n", err)
+				}
+			}
+			continue
+		}
+
 		if err := t.clearCurrentLine(t.wr, t.fd); err != nil {
 			_, _ = fmt.Fprintf(t.errWriter, "write failed: %v\n", err)
 		}