From a customer in Zendesk (loosely paraphrased)

The behavior we would like to have is for coder_app​ resources to not be accessible until after the startup script has completed. To do this, I've set startup_script_behavior​ to "blocking"​ in our templates. The startup_script_behavior documentation, says this means that the workspace is "not ready", which I would expect to mean that you can't yet do things like click on apps.