fix(textinput): slicing outside cap (#532)

* textinput: fix slicing outside cap

* textinput: add test that makes slicing outside cap occur

* chore: tidy with gofmt

---------

Co-authored-by: bashbunni <bunni@bashbunni.dev>

textinput/textinput.go

@@ -700,10 +700,12 @@ func (m Model) View() string {
 func (m Model) placeholderView() string {
 	var (
 		v     string
-		p     = []rune(m.Placeholder)
 		style = m.PlaceholderStyle.Inline(true).Render
 	)
 
+	p := make([]rune, m.Width+1)
+	copy(p, []rune(m.Placeholder))
+
 	m.Cursor.TextStyle = m.PlaceholderStyle
 	m.Cursor.SetChar(string(p[:1]))
 	v += m.Cursor.View()

textinput/textinput_test.go

@@ -30,3 +30,10 @@ func Test_CurrentSuggestion(t *testing.T) {
 		t.Fatalf("Error: expected first suggestion but was %s", suggestion)
 	}
 }
+
+func Test_SlicingOutsideCap(t *testing.T) {
+	textinput := New()
+	textinput.Placeholder = "作業ディレクトリを指定してください"
+	textinput.Width = 32
+	textinput.View()
+}