macOS: What is this mysterious white window? This small white window often pops up and it's very annoying.

I'm using macOS Sierra 10.12.6

It may or may not be related to these questions: 1, 2



The only way to get it to disappear is to click on the highlighted “1”.

But it doesn't stay gone. It appears at random intervals. I can't reproduce it consistently, but every time it appears it is when I press some combination of modifier keys and number keys.

After I click to make it go away it seems not to be able to return for several minutes.

The worst thing is that it seems to be interfering with my ability to use the Ctrl1 keyboard shortcut. Immediately after closing the window I regain my ability to use Ctrl1, but it's not permanent.

I have Japanese text input enabled, and at first I thought maybe it was a text input related window, but now I really have no idea.

Someone in an answer to another question pointed to Microsoft Office as the culprit. I have Office for Mac installed on this machine (not by my choosing), however closing those applications does not make the window disappear. There might be some background process related to Office causing it, but looking through Activity Monitor I can't guess what it might be.

Does anyone know what this is or how to get rid of it?

Thanks
It's the IMK Candidate Window.

Here's what Accessibility Inspector has revealed about it:



Why it has Ctrl 1 and Ctrl 2 assigned to it, and how to disable those shortcuts, I don't know.

But the solution that worked for me was to disable the native macOS input method for Japanese, and install the Google IME instead.

Here are my settings now: