Shiny app with custom domain is in iframe, so how I control the page title?

I have a Shiny app, deployed under the Professional plan and with a custom domain in accordance with the instructions at https://shiny.rstudio.com/articles/custom-domains.html. Everything works fine - except that when viewing the app at the URL I want users to use, the webpage Title (coming from navbarPage(..., windowTitle =""), not that I think that matters) is not showing up in the browser tab. It works fine at the myaccount.shinyapps.io/my-app address, just not at the apps.mydomain.com.au/my-app address I have set up.

Inspecting the source of the page shows that the probable reason is that apps.mydomain.com.au/my-app page is actually an iframe pointing myaccount.shinyapps.io/my-app/. This isn't causing any problems (yet) other than this window title problem, so is there a way I can set the title of the page that is embedding my main location in an iframe?