Setting split-width-threshold no longer prevents windows from being split horizontally

[Ben Key]

Hello,

Today I discovered that setting split-width-threshold no longer
prevents windows from being split horizontally.  The documentation for
split-width-threshold states that "If this is nil, `display-buffer'
cannot split windows horizontally."  However, Emacs still splits the
window horizontally for the completions window and the help window
even with split-width-threshold set equal to nil.  This is a
relatively new problem.