bug#24899: 25.1; x-hide-tip always returns nil with USE_GTK

[martin rudalics]

> I think there might be another problem with the patch though.  There seems
> to be a problem when setting `x-gtk-use-system-tooltips` variable to nil.
>
> I think that `x-show-tip` respects this variable with
>
> `if (x_gtk_use_system_tooltips)`
>
> but `x-hide-tip` does not.  So, the tooltip is created but can't be
> destroyed.

The patch wouldn't change anything in this regard.  The tooltip will be
destroyed as with any other toolkit.  On Emacs-26 you can alternatively
hide it via ‘tooltip-reuse-hidden-frame’.

martin