I do not think this is a problem with UE4, but with how the google IME handles the suggestion box. Unlike the Microsoft one, the Google IME is not integrated with UE4 and has to add the box on top of what you are working on. It does this by getting the bounds of the currently active unit and placing it at the bottom left of it. As such, since the active window is the entire editor the box is placed outside. This might be a simple fix or a difficult one, but that was my two cents.