fix(linux): find the compositor socket when WAYLAND_DISPLAY is not set

`get_wayland_displays` gives up as soon as `Connection::connect_to_env()` fails,
which is any process that was not handed `WAYLAND_DISPLAY`. A login screen is
the case that matters: rustdesk starts the greeter's `--server` without the
compositor variables on purpose, and the desktop layout is then unavailable for
the whole session even though the compositor is running and its socket sits in
the runtime directory of the very user that process runs as.

Measured at an sddm Plasma Wayland greeter: `/run/user/112/wayland-0` exists,
`connect(2)` to it as that user succeeds, and the registry advertises
`wl_output` and `wl_seat`. With this fallback the greeter reads the same output
layout any other session does.

Strictly a fallback after `connect_to_env` has already failed, so it cannot
change a host where that succeeds; where there is no compositor at all the
socket is simply absent and it fails exactly as before.

src/platform/linux.rs

@@ -377,6 +377,47 @@ pub struct WaylandDisplayInfo {
 }
 
 // Retrieves information about all connected displays via the Wayland protocol.
+/// Connect to the compositor socket sitting in `XDG_RUNTIME_DIR` when `WAYLAND_DISPLAY` is not set.
+///
+/// A login screen is the case this exists for. The greeter's `--server` is started deliberately
+/// without the compositor variables, because the DRM capture path talks to the root service and a
+/// render node and never to the compositor -- but the compositor IS running, and its socket is
+/// owned by the very user this process runs as. Measured at an sddm greeter: `/run/user/112/wayland-0`
+/// exists, a plain `connect(2)` as that user succeeds, and the registry advertises `wl_output`.
+///
+/// Without this the output layout is unavailable at a login screen, and everything downstream has
+/// to guess: every DRM display reports origin (0,0) because on Wayland each output scans out of its
+/// own framebuffer, so the uinput pointer range collapsed to a single display on a multi-monitor
+/// greeter.
+///
+/// Strictly a fallback after `connect_to_env` has already failed, so it cannot change any host
+/// where that succeeds; and where there is no compositor at all the socket is simply absent and
+/// this fails exactly as before.
+#[cfg(target_os = "linux")]
+fn connect_to_runtime_dir_socket() -> ResultType<Connection> {
+    use std::os::unix::net::UnixStream;
+    let dir = std::env::var("XDG_RUNTIME_DIR")?;
+    let mut last = None;
+    // Compositors name the socket `wayland-N`; 0 and 1 cover a greeter and a session started after
+    // it. Anything beyond that is not worth probing blind.
+    for name in ["wayland-0", "wayland-1"] {
+        let path = std::path::Path::new(&dir).join(name);
+        if !path.exists() {
+            continue;
+        }
+        match UnixStream::connect(&path).map_err(anyhow::Error::from).and_then(|s| {
+            Connection::from_socket(s).map_err(anyhow::Error::from)
+        }) {
+            Ok(conn) => return Ok(conn),
+            Err(err) => last = Some(format!("{}: {err}", path.display())),
+        }
+    }
+    Err(anyhow::anyhow!(
+        "no usable wayland socket in XDG_RUNTIME_DIR ({})",
+        last.unwrap_or_else(|| "none present".to_owned())
+    ))
+}
+
 pub fn get_wayland_displays() -> ResultType<Vec<WaylandDisplayInfo>> {
     struct WaylandEnv {
         registry_state: RegistryState,
@@ -404,7 +445,11 @@ pub fn get_wayland_displays() -> ResultType<Vec<WaylandDisplayInfo>> {
     sctk::delegate_output!(WaylandEnv);
     sctk::delegate_registry!(WaylandEnv);
 
-    let conn = Connection::connect_to_env()?;
+    let conn = match Connection::connect_to_env() {
+        Ok(conn) => conn,
+        Err(err) => connect_to_runtime_dir_socket()
+            .map_err(|fallback_err| anyhow::anyhow!("{err}; {fallback_err}"))?,
+    };
     let (globals, mut event_queue) = globals::registry_queue_init(&conn)?;
     let queue_handle = event_queue.handle();