In the past there was an issue (reported here) where pressing the
Enter key to select an IME suggestion would also submit the comment.
The good news is that we believe to have found the proper way to handle this, allowing you to use the
Enter key to select your IME suggestion, while also still being able to use
Enter to submit the comment if you're not in the middle of composing a word.
We've enabled this on the Japanese Language Meta site to make sure there are no unforeseen issues (every browser handles IME slightly different) and plan to enable it on the main Japanese Language site next, and after that network-wide.
If you use the IME editor regularly, please feel free to test it out (e.g., on the answer below here) and let me know if there's any issues.
I've written a blog post with the technical details of this change.
Update: This is now live on the main site. Please do still let us know if you see any bugs. Thanks!