title: R Shiny reactive triggered by navigating to particular tabPanel in navbarPage
body: <p>writing with a shiny question. I have a navbarPage, id = "navbar", and in the navbarMenu user can select one among several tabPanels. Each tabPanel is assigned a value (value = 1, value = 2, etc). So input$navbar is reactive value with the value of the selected tabPanel.</p>
  
  <p>I have a reactive expression defined which reacts to the changing of the tabPanel (reacts based on input$navbar). What I actually want is for this to react to navigating to a particular tabPanel, but not navigating <em>away</em> from that tabPanel. So, when input$navbar changes from 1 to 2 I want a reaction, but when changing from 2 to 1 no reaction. How can I achieve this?</p>
  
  <p>Here is relevant snippet of my code, I don't think I need a full reproducible example for this but let me know if I'm wrong.</p>
  
  <pre><code>#ui snippet
  navbarPage(id = "navbar",
          navbarMenu(title = "Title",
                      tabPanel(title = "tp1", value = 1),
                      tabPanel(title = "tp2", value = 2),
                      #more tabPanels and ui stuff...
  
  #server snippet
  rctvfx &lt;- reactive({
                #want this to react only when input$navbar changes from ==1 to ==2
                input$navbar 
                isolate({ 
                    #do stuff 
                })
            })
  </code></pre>