shiny: start the app with hidden tabs, with NO delay

You could use javascript with extendShinyjs() to hide the tabs you want on page load:

Javascript code:

shinyjs.init = function(){
  $('#hello li a[data-value="tab3_val"]').hide();
  $('#hello li a[data-value="tab2_val"]').hide();
}

R code:

ui <- fluidPage(useShinyjs(),
                #Added this js
                extendShinyjs(script = path_to_javascript_file),
                navbarPage("hello", id="hello",
                           tabPanel("home", br(), h3("this is home"),passwordInput("pass", "enter 'password' to see the tabs: "),actionButton("enter", "enter")),
                           tabPanel("tab2", value = "tab2_val", br(), h4("this is tab2")),
                           tabPanel("tab3 with a lot of stuff in it", value = "tab3_val", br(), h4("this is tab3"))))

server <- function(input, output, session) {

  observeEvent(input$enter, {
    if (input$pass == "password"){
      show(selector = '#hello li a[data-value="tab3_val"]')
      show(selector = '#hello li a[data-value="tab2_val"]')
      }})}
shinyApp(ui, server)

---

Alternatively the CSS actually isn't too complicated. If you wanted to go that route you could simply replace the extendShinyjs() call in the above with:

tags$head(tags$style(HTML("#hello li a[data-value = 'tab2_val'], #hello li a[data-value = 'tab3_val'] {
                             display: none;
 }")))

The downside to this is that the formatting of the tabs appears to be off after un-hiding them.

I'd go with renderUI (see @BertilBaron's answer) or appendTab because it's easy to bypass the password when hiding the tabs:

Here is how to achive the desired behaviour via appendTab avoiding the above with basic shiny, no additional JS:

library(shiny)

ui <- fluidPage(navbarPage("hello", id = "hello",
  tabPanel(
    "home",
    br(),
    h3("this is home"),
    passwordInput("pass", "enter 'password' to see the tabs: "),
    actionButton("enter", "enter")
  )
))

server <- function(input, output, session) {
  observeEvent(input$enter, {
    if (input$pass == "password") {
      appendTab(inputId = "hello", tab = tabPanel("tab2", value = "tab2_val", br(), h4("this is tab2")))
      appendTab(inputId = "hello", tab = tabPanel("tab3 with a lot of stuff in it",value = "tab3_val", br(), h4("this is tab3")))
    }
  })
}

shinyApp(ui, server)