It would be great if the frontend would show a browser dialogue message warning that you're about to leave the page whenever you're inside of the form to create a new building block definition. That could be a message like "Are you sure you want to leave the page?"

Since building block definitions can take a significant amount of time to create (considering the definition of inputs/outputs) it would be a shame to lose all progress, e.g. just because you accidently clicked a link in the sidebar.