fix textinput: render full placeholder text

Previously, the placeholder would only display the first character when
Width() was small or unset. This was caused by allocating a rune slice
with size m.Width()+1, which would truncate the placeholder text.

Now we allocate the full placeholder text and display it properly regardless
of the Width() setting.

Fixes #779

Author: RoomWithOutRoof

textinput/textinput.go

@@ -745,8 +745,7 @@ func (m Model) placeholderView() string {
 		render = styles.Placeholder.Render
 	)
 
-	p := make([]rune, m.Width()+1)
-	copy(p, []rune(m.Placeholder))
+	p := []rune(m.Placeholder)
 
 	m.virtualCursor.TextStyle = styles.Placeholder
 	m.virtualCursor.SetChar(string(p[:1]))