I may have resolved this issue with the following patch to
`url-retrieve-synchronously`.
What this achieves is to trigger a `quit` in a controlled environment
rather than allowing it to occur when `accept-process-output` is run.
It's not always wanted to trigger a quit when `(input-pending-p)` is `t`.
But I noticed from placing `while-no-input` around `accept-process-output`
to avoid the `quit` that `url-retrieve-synchronously` would then hang but
with the controlled `quit` happening beforehand, `accept-process-output` no
longer needs `while-no-input` around it. The end result is buttery smooth
helm with no accidental `quit` from typing too fast. I think this may have
resulted in GUI helm faster too.

```
29c29,30
<       (let ((proc (get-buffer-process asynch-buffer)))
---
>       (let ((proc (get-buffer-process asynch-buffer))
>             (counter 0))
71a73,78
>             (with-local-quit
>               (if (input-pending-p)
>                   (progn
>                     (setq counter (1+ counter))
>                     (if (> counter 20)
>                         (keyboard-quit)))))
```
Shane Mulligan

How to contact me:
🇦🇺 00 61 421 641 250
🇳🇿 00 64 21 1462 759 <+64-21-1462-759>
mullikine@gmail.com


On Wed, May 19, 2021 at 11:32 AM Shane Mulligan <mullikine@gmail.com> wrote:

> My apologies. It was literally 5am when I wrote that. I think I have
> misunderstood `C-g` being generated with `quit-flag`. The bad behaviour is
> very clearly still happening. I will try to clarify this problem further by
> experimenting with it. I need to figure out why `while-no-input` suppresses
> the `quit` but merely setting `quit-flag` does not. Something indirect may
> be happening.
>
> Thank you all,
> Shane Mulligan
>
> How to contact me:
> 🇦🇺 00 61 421 641 250
> 🇳🇿 00 64 21 1462 759 <+64-21-1462-759>
> mullikine@gmail.com
>
>
> On Wed, May 19, 2021 at 4:54 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
>> > From: Shane Mulligan <mullikine@gmail.com>
>> > Date: Wed, 19 May 2021 04:32:16 +1200
>> >
>> > Thanks for looking into this so quickly. First some background on the
>> problem. I managed to work through
>> > this issue (https://github.com/emacs-helm/helm/issues/2417) with the
>> `emacs-helm` maintainer and we
>> > found what appears to be that the call to `accept-process-output`
>> inside of `url-retrieve-synchronously` will
>> > generate a `C-g` when there is pending input of any char. As far as I
>> can tell this is an issue only with
>> > terminal emacs. As I understand it, `inhibit-quit`, as used in
>> `accept-process-output` allows a `C-g` to be
>> > propagated outwards and handled and from what I can see by the comments
>> surrounding,
>> >
>> > ```
>> >               ;; accept-process-output returned nil, maybe because the
>> process
>> >               ;; exited (and may have been replaced with another).  If
>> we got
>> >               ;; a quit, just stop.
>> > ```
>> >
>> > the `C-g` in this case is expected.
>> >
>> > But I wonder if `C-g` was meant to be generated if the user was simply
>> mashing keys on the keyboard.  In
>> > this case, the `C-g` emanating from `accept-process-output` was
>> bubbling up into `helm` and `helm` was
>> > treating it like an error.
>> >
>> > Here, you can see a quit being generated from the visual cue in the
>> minibuffer.
>> > https://asciinema.org/a/nAIB8Z1lGgZJqJg9Mt8YiNEM9
>> >
>> > Here, I have added `while-no-input` and I no longer get the `quit`.
>> > https://asciinema.org/a/x9ELZhwDP1IUtmOz0M1cly42H
>> >
>> > However, as I test the addition of `while-no-input` with
>> `helm-google-suggest` (as above), though Quit is no
>> > longer being generated from mashing keys, the key input which would
>> have generated the `quit` is not
>> > immediately shown in `helm`. Instead, it only appears on the next key
>> press. So my implementation may not
>> > be perfect or complete.
>> >
>> > Finding the solution would make the minibuffer far less interrupted
>> while typing when
>> > `url-retrieve-synchronously` is used in the background.
>> >
>> > Thank you.
>> >
>> > Shane Mulligan
>>
>> Please in the future send your responses with the bug address,
>> 48500@debbugs.gnu.org, on the CC list, so that others will see your
>> detailed descriptions.  I won't have time to take a good look into
>> that in the next few days, so it's good to make others aware of your
>> findings, because they might look into it meanwhile.
>>
>> Personally, I find it very strange that typing on the keyboard
>> produces C-g, it shouldn't happen, neither on a TTY nor on a GUI
>> display.
>>
>