Fix IME issue when typing Vietnamese

[hdnha11]

Is this adding or improving a feature or fixing a bug?

This is the bug when typing Vietnamese with macOS IME. Please take a look at the following video:

When typing Vietnamese if the user finishes a word without whitespace or enter (no onCompositionEnd yet) and suddenly change the selection (onCompositionEnd occurred) then no text was inserted, but the view still displays the text which causes that text to disappear if the user does other edit commands (text input, bold,...).

Note: to type "tiếng Việt", turn on macOS Telex IME then type "tieesng Vieejt".

What's the new behavior?

User can input Vietnamese using macOS IME like this:

How does this change work?

Based on @yunwuxin work (fixed the cursor when typing and flashing when IME on) I add one more command insertText when onCompositionEnd occurred to explicitly sync the text in the view with the editor state.

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Based on the work of @yunwuxin in #2415

[icarusysuper]

It seems that this PR will solve IME problem @ianstormtaylor

[minwe]

Hi, @hdnha11 I'm tring to resolve Chinese IME problem with your approach.

It works, but it caused new problem:

• Click CMD + B
• Input some text with IME, it will insert duplicated text.

Any idea about this?

[arkee11]

嗨，@ hdnha11我正努力用您的方法解决中文IME问题。

它有效，但是引起了新的问题：

• 点击CMD + B
• 使用IME输入一些文本，它将插入重复的文本。

有什么想法吗？
hi~遇到了同样的问题，请问有解决思路吗？

[ianstormtaylor]

Fixed by #3093.