1910016 - `TextEditor` should manage its focus state with a `bool` flag

Status: ASSIGNED

(Core :: DOM: Editor, task)

Description

[Masayuki Nakano [:masayuki] (he/him)(JST, +0900)(PTO: 6/26-6/30))]

As you know, focus state of editor class does not match nsFocusManager state if focus or blur event is fired asynchronously. Therefore, there is no way to check whether an editor gets received the latest focus or blur. So, like we do it in bug 1798379 for HTMLEditor, TextEditor should also have the flag (in EditorBase).