Select all in textBox uses the length of the rune slice

internal/system/textbox.go

@@ -392,6 +392,8 @@ func (this *textBox) runeUnderMouse () int {
 	return this.drawer.AtPosition(fixPt(position))
 }
 
+// TODO the keynav here should make better use of input key chords.
+
 func (this *textBox) handleKeyDown (key input.Key, numberPad bool) bool {
 	if this.box.handleKeyDown(key, numberPad) { return true  }
 	if !this.selectable                       { return false }