This only happens on Mac and WebView (HTMLEditor, WebView text box). Not a regression. TextField and TextArea are not affected.
Steps to reproduce:
1. Focus on a HTMLEditor, invoke IME for Japanese.
2. Type string "suzuki".
3. Press "Space", the string is converted to Japanese characters.
4. Press "Space" again, the candicate windows is displayed.
5. Press "Return" to confirm the selection.
6. Press "Return" to commit the Japanese text.

There is nothing got committed. The composing text is removed, and the caret moves to the next line.
If press "Return" after step 3, the converted text can be commited. So this issue is related to the candidate window.