title: observeEvent triggered by events in the handling expression, rather than just the eventExpr trigger
body: <p>The handling expression in the <code>observeEvent</code> in this reprex at first waits for the trigger button, but then is run whenever the input <code>x</code> in the 'Value&quot; field is changed, not waiting for the trigger button.</p>
  <pre><code>library(shiny)
  
  ui = fluidPage(
      column(12,
                   textInput(&quot;x&quot;, &quot;Value&quot;,&quot;starting_text&quot;),
                   br(),
                   actionButton(&quot;button&quot;, &quot;trigger eventReactive!&quot;),
                   h3(&quot;The textOutput below should only update when you click the button but after the first click it reacts directly to the textInput field&quot;),
                   textOutput(&quot;out_text&quot;)
      )
  )
  
  server = function(input, output,session) {
      
      observeEvent(input$button, {
          output$out_text &lt;-  renderText( input$x)
      })
      
  }
  
  shinyApp(ui,server)
  
  </code></pre>
  <p>I can solve this problem using an <code>eventReactive</code> or answers from this  very similar  Stack Overflow question:
  <a href="https://stackoverflow.com/questions/51201127/r-shiny-observeevent-continues-to-trigger">R Shiny observeEvent continues to trigger</a></p>
  <p>There are answers there solving this problem by assigning <code>input_value</code> to a dummy variable inside of the handling expression, and by using <code>isolate()</code> around <code>input$input_value</code> with no  for why they work. I can also solve this problem with an intermediate  <code>eventReactive</code>.</p>
  <p>So I'm still not satisfied.  I think that my reprex and the one on the other question should just work as posted.</p>
  <p>So either there is a fundamental bug in shiny, or as is usually the case, I'm misunderstanding something deeper.
  Or maybe this is a bug, but fixing it would break existing shiny apps that rely on this behavior?</p>
  <p>To sum up:
  Can someone explain why the eventReactive gets run even though the triggering event does not occur?
  Can someone also explain why the two answers on StackOverflow work, especially the one with the  assignment to the Dummy variable?</p>
  <p>Finally I'll mention that there may be other questions here and on StackOverflow regarding this issue, but I did not find the reprexes to be sufficiently minimal or easy to digest</p>