bug#10945: Can't search through help menu.

[Juri Linkov]

>> >> emacs -Q
>> >> C-h ?       ; brings up the help menu
>
>> >> I now want to search through this menu for "ChangeLog":
>
>> >> C-x         ; intending C-x o
>
>> >> The menu disappears.  This isn't friendly.
>
>> > We can now search that window using `C-h ? C-s', and it switches to
>> > the *Metahelp* window.  This functionality was added in this comit:
>
>> > 956c39d11d 2018-04-02 Make help-for-help window searchable (Bug#19655)
>
>> > `C-h C-x o' still doesn't work, though.
>
>> > Is that good enough here?
>
>> Maybe help-for-help should use the minibuffer for reading commands
>> instead of using explicit read-key-sequence?  This would allow
>> switching to the *Metahelp* window with 'C-x o'.
>
> This sounds like an excellent idea.

I tried, but the major problem is how to create a wrapper around
commands in 'help-map' to exit the minibuffer before running them.
For example, evaluating:

(read-from-minibuffer "C-h (Type ? for further options)- " nil help-map)

then typing one of the help character, doesn't exit the minibuffer.
Some additional layer of indirection could exit the minibuffer
before running the command, but currently I have no idea how to do this.

PS: Also isearch-help-for-help can't use the minibuffer to read help keys
while isearch-mode is enabled, but this is a minor obstacle.