

I’ve been using it for a while, haven’t noticed the candidate box margins being weird until you mentioned it.
You can make the font for the candidate box larger to fill the unoccupied space, but that sounds like a bad solution.
It’s developer opened an issue for improving that part of the keyboard not that long ago, probably there is something in the works. You may want to voice your concerns there.

Precisely the reason why I’m not using Fcitx, on a touchscreen there is nothing better than flick input for Japanese.