Why shiny port does not update until I run twice in RStudio?

Question

My shiny app consists of global.R, server.R and ui.R. I used "Run App" button in RStudio (when I am at global.R) to launch the app.

When I try to specify the port number, e.g. 8080 by adding options(shiny.port = 8080) in global.R. When I click "Run App" once, it doesn't immediately use the 8080 port, instead it still uses random available port. But if close the app and click "Run App" again, it now uses the 8080 port. I wonder how I can let app use the port 8080 in the first place?

Thank you!