bug#68927: CapsLock gets toggled by a frozen Emacs even if that's supposed to be prevented by a third party app

[Nikolay Kudryavtsev]

I've originally mentioned this in discussion on #68914, because it 
seemed like a somewhat similar problem. But Eli suggested there that I 
should report this separately.

Let me disclaim in advance that I believe that this is an extremely 
minor niche issue and the value of investigating it is only in making 
sure that there aren't any other, more important use cases hit by this.

So here are the detailed reproduction steps:

1. Use Windows. I haven't really tested other platforms for this, but I 
highly suspect that this is a platform-specific issue.

2. Setup Mahou layout switcher. You can download it here:

https://gitea.com/BladeMight/Mahou/releases/download/latest-commit/Release_x86_x64.zip

The default settings should be sufficient. But to make sure, here are 
the relevant ones:

-On the "Layouts" tab, make sure "Change to specific layout by keys" is 
set to on and Caps Lock is the first key.

-On the "Functions" tab make sure "Remap Caps Lock to F18" is set to on.

3. Run Emacs -q

4. Eval (global-set-key [f18] nil) . We're doing this to avoid extra 
paste due to Mahou F18 setting. But strictly it's not necessary.

5. Freeze Emacs by forcing it to run some code for a few seconds. I've used:

(let ((x 0))
  (dotimes (i 10000000)
    (setq x (1+ x))))

6. While the above is running, start tapping CapsLock. You'd see that 
CapsLock toggles even though it shouldn't and this only happens when 
Emacs is frozen.

Few other notes regarding this bug:

Mahou is somewhat buggy too, I've seen it pointlessly toggle caps in 
other apps, but it does not seem to be the same issue.

I originally noticed it while using a certain proprietary app for the 
same purpose. So there are at least 2 applications with which this 
reliably happens.

There are many more applications with which this is not reproducible. I 
could not reproduce it with AutoHotkey, a dozen of other apps, including 
some other layout switchers like dotSwitcher.

So it seems like the third party app should be intercepting CapsLock in 
a very particular way for this to show up and thus the importance of the 
Mahou F18 setting is particularly interesting, because without it, the 
bug is not reproducible with Mahou.

I was wrong about Emacs 25 not suffering from this problem. I could 
reproduce it there too.