It's possible to resize the candidate window in such a way that a selection at the very bottom is impossible to read -- it's only visible as a thin blue bar.
Resize the window so that only the top pixels of any capital letters are visible on the bottom line. Then use the keyboard to scroll downwards. When it gets to the bottom line, you won't see any of the selected text.
Resize the window so that only the top pixels of any capital letters are visible on the bottom line. Then use the keyboard to scroll downwards. When it gets to the bottom line, you won't see any of the selected text.