"Shift+X" didn't actually map any key, as opposed to "Shift+x". This is
because shift usually changes the case of a character, so a plain
printable character like "X" simply can never be combined with shift.
But this is not very intuitive. Always remove the shift code from
printable characters. Also, for ASCII, actually apply the case mapping
to uppercase characters if combined with shift. Doing this for unicode
in general would be nice, but that would require lookup tables. In
general, we don't know anyway what character a key produces when
combined with shift - it could be anything, and depends on the keyboard
layout.
wm4