Shiny 1.8.1 seems to break reactivity between my linked modules about shiny

[cpsievert]

Also, we'll be submitting a new version of Shiny to CRAN today that downgrades this error to a warning (unless shiny::devmode() is enabled). In the meantime, you can install the development version:

remotes::install_github("rstudio/shiny")

[gadenbuie]

Thank you both for the report. Both cases are likely the result of duplicate input or output IDs. We recently introduced in 1.8.1 a developer console that surfaces duplicate IDs, but it unintentionally broke apps that previously worked (with potentially undefined behavior). #4019 fixes this issue by allowing apps with duplicate IDs to work as before -- we'll be releasing a version of shiny that includes this fix soon.

In the mean time, you can verify that you're encountering this issue by running your app with Shiny's dev mode enabled. Simply call shiny::devmode() before running the app. In this mode, a client-side error console will surface any runtime errors found by Shiny. In @john-harrold's case the app will show you that the FG-ui_fg_msg output ID is duplicated in Plot panel.

Alternatively, you can open your app in a browser window. Then open the Console tab in the developer tools (right click on the page and choose "Inspect" or "Inspect Element" to open the browser's dev tools). There you'll see an error message like this:

[john-harrold]

OK I feel rather foolish. It looks like I found this before but didn't change it. I probably thought I put it there on purpose :)

Also shiny::devmode() is cool.

Thank you for your quick response. I'll go submit an update to CRAN now.

[Martin-Jung]

Don't have a reproducible example, but just to confirm that this happens to my app as well (windows and linux). After updating to shiny 1.8.1 the app simply does not load anymore, getting stuck at the preloader. Using a bs4dash app.
Had to downgrade to shiny 1.8.0 to make it work again.