Drop recordings (and blueprints) on separate threads.

This prevents the UI from hanging while dropping a large recording,
which is useful when the user is trying to close multiple recordings
at once.

If threads are not available, automatically falls back to dropping on
the current thread.

Author: kpreid

crates/viewer/re_viewer_context/src/store_hub.rs

@@ -340,6 +340,21 @@ impl StoreHub {
                     .retain(|_, id| id != store_id);
             }
         }
+
+        // Drop the store itself on a separate thread,
+        // so that recursing through it and freeing the memory doesn’t block the UI thread.
+        #[allow(
+            clippy::allow_attributes,
+            clippy::disallowed_methods,
+            reason = "If this thread spawn fails due to running on Wasm (or for any other reason),
+                      the error will be ignored and the store will be dropped on this thread."
+        )]
+        let (Ok(_) | Err(_)) = std::thread::Builder::new()
+            .name("drop-removed-store".into())
+            .spawn(|| {
+                re_tracing::profile_scope!("drop store");
+                drop(removed_store);
+            });
     }
 
     pub fn remove(&mut self, entry: &RecordingOrTable) {