The frame is splitted into two windows, one for *scratch* and another
for *Help*, "foo" is gone, C-x 1 does not work from *scratch*, C-x 1
from the *Help* buffer reports
byte-code: Cannot make side window the only window
>> displayed on a regular window covering half the screen with its usual
>> modeline. That window wont go away easily (C-x 1 while the other window
>> is active has no effect).
> That's part of the idea. C-x 1 should not delete such windows and
> should not make such a window the only one on its frame.
It seems that Emacs reuses that window for displaying other buffers
(*Help*, in the recipe described above)