bug#34517: tmm menubar menu items have no effect on Android

[Juri Linkov <juri@linkov.net>]

>> > Touching the top menu items like “File” opens a file submenu nicely
>> > in tmm's *Completions* buffer.
>> >
>> > But touching menu items in its submenu like “New Window on Right”
>> > does nothing.
>>
>> I can reproduce this in the latest version 27 on GNU/Linux:
>> clicking with the mouse on menu items from M-` (tmm-menubar)
>> has the same effect, i.e. no effect.
>
> tmm-menubar is supposed to be for when there's no mouse at all, so why
> do we expect a mouse click to do anything in that case?

Smartphones translate screen touch events to click event, so this is
the only way to use menus.  Also the help text of tmm menus says:

  "Click on a completion to select it."

But now I see that tmm relies on completing-read-default
and inserts initial input that gets concatenated with
an item selected by clicking in the *Completions* buffer.

When initial input is deleted manually with e.g. <backspace>
before clicking on a menu item, then tmm works correctly.

So the bug is in completing-read-default and can be reproduced
with a simpler test case:

0. emacs -Q

1. ‘C-h f TAB’ displays a list of completions

2. type a nonexistent function name, i.e. some random text
   in the minibuffer, e.g. “blabla”

3. click on an existing valid completion in the *Completions* buffer,
   e.g. on “append”

4. instead of getting the selected item “append”, it fails with:

   user-error: Symbol’s function definition is void: appendblabla