library(shiny)

ui <- fluidPage(
  theme = bslib::bs_theme(version = 4),
  tabsetPanel(
    id = "tabs",
    tabPanel("A", "a"),
    navbarMenu("menu", tabPanel("B", "b"))
  )
)

server <- function(input, output, session) {
  observe({
    print(input$tabs)
  })
}

shinyApp(ui, server)