Handle kernel messages synchronously A problem can happen when two messages come in for different comms, where the second depends on the first (for example, the first might be a message setting the state of a widget, and the second triggering a view creation for the widget). Since comm message queues are independent of each other, the second message could be executed before the first message. This exposes a more fundamental assumption users are likely to have that messages from python are processed synchronously. Thanks to @dmadeka for reporting an error that led to discovering this issue.