The problem:
When using bslib::navset_tab(), each tab panel is dynamically created and destroyed as you switch between them. This means that any inputs (like selectInput()) in inactive tabs are removed from the DOM. As a result, trying to update them from the server with updateSelectInput() or similar functions fails or causes infinite loops.

navset_tab(
  nav_panel("Panel 1", selectInput("filter_ip1", ...)),
  nav_panel("Panel 2", selectInput("filter_ip2", ...))
)

Here, only one of these selectInput() elements exists at a time. When you switch tabs, the other is destroyed and recreated later. So if you try this:

observeEvent(input$filter_ip1, {
  updateSelectInput(session, "filter_ip2", selected = input$filter_ip1)
})

it won’t work — because when Panel 1 is active, filter_ip2 simply doesn’t exist.

The fix — keep your tabs alive

bslib has an option called keep_alive that tells Shiny to keep all tab panels in memory instead of destroying them when hidden. This allows inputs to persist and be updated normally.

navset_tab(
  id = "main_tabs",
  keep_alive = TRUE,
  nav_panel("Panel 1", selectInput("filter_ip1", "IP to exclude (panel 1):", choices = NULL)),
  nav_panel("Panel 2", selectInput("filter_ip2", "IP to exclude (panel 2):", choices = NULL))
)

Now the synchronization works perfectly:

observeEvent(input$filter_ip1, ignoreInit = TRUE, {
  updateSelectInput(session, "filter_ip2", selected = input$filter_ip1)
})

observeEvent(input$filter_ip2, ignoreInit = TRUE, {
  updateSelectInput(session, "filter_ip1", selected = input$filter_ip2)
})

Result

• Inputs are no longer destroyed when switching tabs
• updateSelectInput() and observeEvent() work normally
• Values persist across panels — no infinite loops

Summary

TL;DR: If your Shiny app uses bslib::navset_tab() and you need to share state or update inputs across tabs, add keep_alive = TRUE and everything will just work.