bug#49714: 28.0.50; TRAMP burns CPU and has insufficient user reporting when using xxxx-sk SSH keys

[Dima Kogan]

Hi Michael. I just updated my emacs to a recent-enough build that has
all the fixes: 0b049fe71d7 from 2021/08/06. I see the issue I described
earlier. Specific steps:

1. emacs -Q '/ssh:whatever:'

2. emacs asks for the passphrase. I enter it

3. emacs says "Confirm user presence..." in the minibuffer

I can touch the yubikey, and emacs completes the sequence. While it's
waiting for the yubikey to be touched, emacs is frozen. The toolbar is
rendered, but the modeline and scroll bars are not. The point is not
rendered. If I switch to another window, and switch back, the toolbar is
redrawn, but nothing else is. Most notably, the "Confirm use
presence..." text in the minibuffer is not redrawn.

I can follow slightly different steps:

1. emacs -Q

2. C-x C-f /ssh:whatever:

3. emacs asks for the passphrase. I enter it

If I do this instead, emacs freezes right away, and I don't ever see the
prompt in the minibuffer at all. Again, it's not "frozen", since
touching the yubikey works properly, but the UI indication is missing.

You don't see this?

Thanks!