Using shiny 1.6.0 and bslib 0.3.0.

This work as expected:

library(shiny)

ui <- navbarPage(
    title = "Minimal Example Issue",
    id = "tabs",
    header = conditionalPanel(
            condition = "input.tabs == 'tab_1'",
            HTML("Special tab 1")
        ),
    tabPanel("tab_1", "Hello"),
    tabPanel("tab_2", "Hi")
)

server <- function(input, output) {}

shinyApp(ui = ui, server = server)

This doesn't as the condition isn't met on selection of tab_1:

library(shiny)
library(bslib)

ui <- page_navbar(
    title = "Minimal Example Issue",
    id = "tabs",
    header = conditionalPanel(
            condition = "input.tabs == 'tab_1'",
            HTML("Special tab 1")
        ),
    nav("tab_1", "Hello"),
    nav("tab_2", "Hi")
)

server <- function(input, output) {}

shinyApp(ui = ui, server = server)

I would like to understand which JS element has replaced the input.tabs in the second example.