@alans Yes, Shiny reactive functions really strain my brain! I've created a multi-page Shiny app for work, organised into modules. It's fun to write, but much more challenging to test and debug than RMarkdown documents for sure. Over the time I've been building it, I've found that building Shiny apps as R packages and ruthlessly separating the server and UI functions from all the other code makes it easier to test the code outside of the shiny environment.