backend/sys: associate imported proxy with correct event queue

fixes a regression introduced in e777e85
which moved all proxies to a separate event queue but left proxies imported
with `Backend::manage_object` on the default queue.

wayland-backend/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+### Bugfixes
+
+- backend/sys: Fix importing external objects with `Backend::manage_object` by
+  associating the proxy with the correct event queue
+
 ## 0.3.6 -- 2024-07-16
 
 ### Bugfixes

wayland-backend/src/sys/client_impl/mod.rs

@@ -757,14 +757,22 @@ impl InnerBackend {
     }
 
     /// Start managing a Wayland object.
+    ///
+    /// Safety: This will change the event queue the proxy is associated with.
+    /// Changing the event queue of an existing proxy is not thread-safe.
+    /// If another thread is concurrently reading the wayland socket and the
+    /// proxy already received an event it might get enqueued on the old event queue.
     pub unsafe fn manage_object(
         &self,
         interface: &'static Interface,
         proxy: *mut wl_proxy,
         data: Arc<dyn ObjectData>,
     ) -> ObjectId {
         let mut guard = self.lock_state();
-        unsafe { self.manage_object_internal(interface, proxy, data, &mut guard) }
+        unsafe {
+            ffi_dispatch!(wayland_client_handle(), wl_proxy_set_queue, proxy, guard.evq);
+            self.manage_object_internal(interface, proxy, data, &mut guard)
+        }
     }
 
     /// Start managing a Wayland object.