bug#34819: 26.1; Blank help-echo tooltips for mode line menus

[mituharu]

> On Thu, 20 Jun 2019 14:01:54 +0900,
> YAMAMOTO Mitsuharu wrote:
>>
>> The contents of the tooltip seems to be usually shown by the
>> flush_dirty_back_buffers call from handle_one_xevent.  But the control
>> does not go back to read_socket_hook during menu tracking, so
>> flush_dirty_back_buffers is not called in such a case.
>
> The argument was too rough, actually.  During menu tracking,
> handle_one_xevent is called via popup_get_selection and
> x_dispatch_event.  Difference should come from other places.

It turns out the contents of the tooltip is usually shown by
unblock_buffer_flips in double-buffer setting.  Calls to
flush_dirty_back_buffers do not contribute to tooltip display
because FRAME_GARBAGED_P is set for the tooltip frame.

I think the patch I posted previously does the right thing.
Please try it.

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp