GTK4 and ext-session-lock-v1

[happenslol]

Hi everyone!

I'm looking to implement a screen locker with the ext-session-lock-v1 protocol using GTK4, and I thought this might be a good place to get some help since this project also had to work around the fact that custom surface support was removed.

The session lock protocol requires the user to pass a surface for each screen, which will be exclusively displayed on that screen. In GTK3, that was possible like this (in rust):

let wl_window = gtk_window.downcast::<gdkwayland::WaylandWindow>().unwrap();
gdk_wayland_sys::gdk_wayland_window_set_use_custom_surface(wl_window.to_glib_none().0);
let wl_surface = get_wl_surface(&wl_window); // omitted helper function
session_lock.create_lock_surface(surface, &output, &queue_handle);

Since gdk_wayland_window_set_use_custom_surface has been removed in GTK4, I've had a look at the code to figure out what it actually does. The only 2 places that are affected by the use_custom_surface flag in GTK3.24 are window mapping (which is aborted early if the flag is set) and should_inhibit_size (which is always FALSE if the flag is set). I feel like it should somehow be possible to prevent these 2 things in GTK4 as well.

It seems from reading the discussions on the gtk3 repo for this library that a lot of the difficulty with porting this library came from the fact that

1. GTK4 creates an XDG surface automatically, which needs to be prevented, and
2. Popups are difficult to handle (not sure if I understood this correctly).

2 is definitely not a problem, since the session lock protocol only allows one surface per screen, which rules out popups in general. I haven't tested yet if the wayland client libraries are fine with receiving an XDG surface, I'll do that later today.

Does anyone have any thoughts on how this could work? Could techniques from this library be used to help me? Are there other effects of use_custom_surface that I've missed?

[wmww]

Hi! Seems like you're on the right track. The main issue with using the session lock protocol with GTK is indeed stopping GTK from creating an XDG shell surface. I strongly suggest using the approach this library does, as explained here and mostly implemented here.

Since ext-session-lock-v1 is a well established and useful protocol, I'd consider adding support for it to this library. Not sure if I'll get around to it any time soon, but PRs absolutely welcome.