Fix re-rendering when prompt is exactly the width of the terminal

go.mod

@@ -5,7 +5,7 @@ require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/hinshun/vt10x v0.0.0-20180616224451-1954e6464174
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51
-	github.com/kr/pty v1.1.4 // indirect
+	github.com/kr/pty v1.1.4
 	github.com/mattn/go-colorable v0.1.2 // indirect
 	github.com/mattn/go-isatty v0.0.8
 	github.com/mgutz/ansi v0.0.0-20170206155736-9520e82c474b

renderer.go

@@ -148,8 +148,15 @@ func (r *Renderer) countLines(buf bytes.Buffer) int {
 			delim = len(bufBytes) // no new line found, read rest of text
 		}
 
-		// account for word wrapping
-		count += int(utf8.RuneCount(bufBytes[curr:delim]) / w)
+		if lineWidth := utf8.RuneCount(bufBytes[curr:delim]); lineWidth > w {
+			// account for word wrapping
+			count += lineWidth / w
+			if (lineWidth % w) == 0 {
+				// content whose width is exactly a multiplier of available width should not
+				// count as having wrapped on the last line
+				count -= 1
+			}
+		}
 		curr = delim + 1
 	}
 

renderer_posix_test.go

@@ -0,0 +1,91 @@
+// +build !windows
+
+package survey
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/AlecAivazis/survey/v2/terminal"
+	pseudotty "github.com/kr/pty"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRenderer_countLines(t *testing.T) {
+	t.Parallel()
+
+	termWidth := 72
+	pty, tty, err := pseudotty.Open()
+	require.Nil(t, err)
+	defer pty.Close()
+	defer tty.Close()
+
+	err = pseudotty.Setsize(tty, &pseudotty.Winsize{
+		Rows: 30,
+		Cols: uint16(termWidth),
+	})
+	require.Nil(t, err)
+
+	r := Renderer{
+		stdio: terminal.Stdio{
+			In:  tty,
+			Out: tty,
+			Err: tty,
+		},
+	}
+
+	tests := []struct {
+		name  string
+		buf   *bytes.Buffer
+		wants int
+	}{
+		{
+			name:  "empty",
+			buf:   new(bytes.Buffer),
+			wants: 0,
+		},
+		{
+			name:  "no newline",
+			buf:   bytes.NewBufferString("hello"),
+			wants: 0,
+		},
+		{
+			name:  "short line",
+			buf:   bytes.NewBufferString("hello\n"),
+			wants: 1,
+		},
+		{
+			name:  "three short lines",
+			buf:   bytes.NewBufferString("hello\nbeautiful\nworld\n"),
+			wants: 3,
+		},
+		{
+			name:  "full line",
+			buf:   bytes.NewBufferString(strings.Repeat("A", termWidth) + "\n"),
+			wants: 1,
+		},
+		{
+			name:  "overflow",
+			buf:   bytes.NewBufferString(strings.Repeat("A", termWidth+1) + "\n"),
+			wants: 2,
+		},
+		{
+			name:  "overflow fills 2nd line",
+			buf:   bytes.NewBufferString(strings.Repeat("A", termWidth*2) + "\n"),
+			wants: 2,
+		},
+		{
+			name:  "overflow spills to 3rd line",
+			buf:   bytes.NewBufferString(strings.Repeat("A", termWidth*2+1) + "\n"),
+			wants: 3,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			n := r.countLines(*tt.buf)
+			assert.Equal(t, tt.wants, n)
+		})
+	}
+}