Re: cursor-color frame parameter cannot equal background-color

[Richard Stallman]

    Put these sexps in buffer *scratch*:

    1. (setq default-frame-alist '((foreground-color . "Blue")
                                   (mouse-color . "Red")
                                   (cursor-color . "Red")))

    2. (modify-frame-parameters (selected-frame)
                                '(;;(background-color . "Black")
                                  (cursor-color . "Black")))

    Eval #1. C-x 5 2 to create a new frame. The new frame's cursor color
    is Red

    Eval #2 (in the new frame). The new frame's cursor is now black - no
    problem. Delete the new frame.

    Uncomment the background-color in #2.

    Eval #1 again and C-x 5 2 to create a new frame.

    Eval #2 (in the new frame). The new frame's cursor is still red -
    the spec change had no effect in this regard. This is the main bug.

It is not a bug, it is intentionally coded in x_set_cursor_color.  It
is trying to avoid making the cursor invisible because of a careless
choice of colors.

    An application where this has a negative effect uses space characters
    with a face that has a background different from the frame background,
    to make spaces visible in different ways. The application wants the
    cursor color to be the same as the frame's background color
    (black).

I do not understand why it wants this.  Can you explain?