Merge pull request godotengine#96904 from RandomShaper/fix_previewer_deadlock

ResourceLoader: Fix deadlocks caused by the resource changed feature

Author: akien-mga

core/io/resource_loader.cpp

@@ -860,7 +860,7 @@ Ref<Resource> ResourceLoader::_load_complete_inner(LoadToken &p_load_token, Erro
 						}
 					}
 					core_bind::Semaphore done;
-					MessageQueue::get_main_singleton()->push_callable(callable_mp(&done, &core_bind::Semaphore::post));
+					MessageQueue::get_main_singleton()->push_callable(callable_mp(&done, &core_bind::Semaphore::post).bind(1));
 					done.wait();
 				}
 			}

editor/editor_resource_preview.cpp

@@ -533,8 +533,10 @@ void EditorResourcePreview::stop() {
 			}
 
 			while (!exited.is_set()) {
+				// Sync pending work.
 				OS::get_singleton()->delay_usec(10000);
-				RenderingServer::get_singleton()->sync(); //sync pending stuff, as thread may be blocked on rendering server
+				RenderingServer::get_singleton()->sync();
+				MessageQueue::get_singleton()->flush();
 			}
 
 			thread.wait_to_finish();