bug#69083: Emacs's keyboard hook state is not reset on session lock (Windows)

[Raffael Stocker]

Hi,

when locking the computer using WIN+L on Windows, Emacs is supposed to
reset its keyboard hook state.  This is done in the function
‘reset_w32_kbdhook_state’ in ‘w32fns.c’, which is called upon receiving
the ‘WM_WTSSESSION_CHANGE’ message with the ‘WTS_SESSION_LOCK’
parameter.  However, this message is never received, because to receive
it, an application must register for session notifications, which Emacs
doesn't do.

When Emacs can't reset its state, the handling of the windows key is
confused after locking and unlocking the computer, but the issue seems
to auto-correct when a windows key is pressed again.  I didn't observe
any real problem with it, but I have not configured any non-default
handling of the windows keys in Emacs.  It may be different in those
cases.

Note that while this bug looks like it might be connected to bug#68914,
I believe it's a separate issue.  The behaviour described there is
neither triggered by this bug nor solved by the attached patch.

The attached patch rectifies the situation, but to do so, Emacs must be
linked with ‘wtsapi32.dll’.  I am not sure whether there is a different
way of resetting the state that doesn't require the introduction of a
new dependency.

To receive session notifications, one must provide a window handle,
which is fine if Emacs does not run in console mode.  I don't know
whether it is possible to get these notifications in console Emacs; at
least using the console handle didn't work for me.

I also noticed that while the keyboard hook is set up in console mode
using ‘setup_w32_kbdhook’, there does not seem to be a corresponding
call to ‘remove_w32_kbdhook’.  Also, in console mode the keyboard hook
is always installed, while in GUI Emacs it is only installed when
‘WINDOWSNT’ is defined.

I have zero experience in Windows programming, so it would be highly
desirable if someone with more knowledge could comment on the issue and
my proposed solution.

Regards,
Raffael

reset_kbdhook.patch
Description: session notification patch