Guest sees multiple caps_lock events when caps_lock is remapped to ctrl on the client

Originally reported against virt-viewer

https://gitlab.com/virt-viewer/virt-viewer/-/issues/29

Running on Fedora 33, with GNOME desktop, launch gnome-tweaks

Under additional keyboard options, Caps lock behaviour, select Caps lock is also ctrl.

Now attempt to interact with a guest, pressing Caps lock.

Since SPICE is forwarding raw scancodes, the guest is still going to see a "Caps lock" scancode of course.

The wierd part is that a press+release on the client results in press+release, then a short delay, then press+release in the guest.

It looks like the remapping of capslock to be a modifier in the client is confusing spice-gtk somewhere, causing it to inject an extra set of input events.

This happens on any other Caps Lock mapping options other than gnome-tweaks 'Make Caps Lock an additional Ctrl', and even with Xmodmap.

Perhaps this is caused by the automatic syncing of modifiers. How about adding an option to disable modifier syncing? This could be a feature that privacy-focused projects like Whonix would appreciate.

Additionally, I want to mention a related behavior. I had the Caps Lock and Escape keys unmodified in my host, but in the guest, I enabled the 'swapescape' option (with setxkbmap -option caps:swapescape). Every time I pressed Escape, xev showed four events, two of which were key presses and releases with keycode 66 (which is usually the keycode for the Caps Lock key, not for Escape, which is the key I pressed).

There is code relevant to this on src/spice-gtk-session.c, under the comment Automatically sync modifiers (Caps, Num and Scroll locks) with the guest., and in the function get_keyboard_lock_modifiers

Adding an option to disable modifier syncing sounds reasonable to me.

Greetings, i'm facing a similar issue, using caps to switch keyboard layouts both on a host machine and a libvirt VM accessed via spice-gtk, and the caps events are duplicated.

This is solved quite easily by forcing a return in spice_gtk_session_sync_keyboard_modifiers_for_channel. In my case it worked perfectly with no adverse side effects. Perhaps adding an option to control sync_modifiers is straightforward.

--- spice-gtk-0.42/src/spice-gtk-session.c      2023-01-23 11:04:11.000000000 +0100
+++ /root/spice-gtk-session.c   2023-11-20 18:11:35.909577555 +0100
@@ -153,7 +153,8 @@
 
    g_return_if_fail(SPICE_IS_INPUTS_CHANNEL(inputs));
 
-    if (SPICE_IS_GTK_SESSION(self) && !self->priv->sync_modifiers) {
+    /* if (SPICE_IS_GTK_SESSION(self) && !self->priv->sync_modifiers) { */
+    if (TRUE) {
        SPICE_DEBUG("Syncing modifiers is disabled");
        return;
    }

On wayland environnement, it seems to have a big side effect of desynchronisation between capslock / numlocks between vm and host

Hi all. Any progress on this issue? It would be great help if it would get fixed, it is really really annoying to have to work with a backspace that randomly deletes chars. Thanks in advance.

+1. It's hard to break my capslock-as-escape habit in vim :(