This is only reproducible on Windows8. It seems there is a new feature for Japanese IM for Win8 that when composing, a candidate window (maybe high frequency candidates) is displayed without pressing SpaceBar. Up and Down arrow key can select the candidates in the window.

There are 2 problems comparing with Notepad:
1. The candidate window is not aligned with composing text.
2. When pressing Up and Down arrow key, the candidate window moves.

For example, type "haruka", the candidate window is displayed around the last character of composed text, and when pressing Down arrow key, the window moves to the first character of composed text.

2 videos are attached for javafx and notepad.