title: Shiny SelectInput and SelectizeInput
body: <p>I updated my Shiny library to version 1.1.0 and I noticed some very strange behavior with selectInput/selectizeInput and observeEvent/eventReactive.</p>
  
  <ol>
  <li><p>The problem occurs when I press backspace and clear the contents of the drop-down menu. In the previous Shiny version the backspace coupled with a eventReactive, the reactive expression wouldn't evaluate (I guess it treated this as a NULL) observe and reactive would evaluate which is desired.</p></li>
  <li><p>req() is also behaving weird, in Example 1 below if we press the backspace and clear the input, renderTable is triggered but when req(input$variable) is empty the table disappears. In the previous version if Shiny I believe the table would simply remain the same.</p></li>
  </ol>
  
  <p>Reproducing Code:</p>
  
  <p>Example 1</p>
  
  <pre><code>shinyApp(
    ui = fluidPage(
      selectizeInput("variable", "Variable:",
                  c("Cylinders" = "cyl",
                    "Transmission" = "am",
                    "Gears" = "gear")),
      tableOutput("data")
    ),
    server = function(input, output) {
      observeEvent(input$variable,{
        cat("Printing: ",input$variable,"\n")
      })
  
      output$data &lt;- renderTable({
        req(input$variable)
        Sys.sleep(2)
        mtcars[, c("mpg", input$variable), drop = FALSE]
      }, rownames = TRUE)
    }
  )
  </code></pre>
  
  <p>or</p>
  
  <p>Example 2</p>
  
  <p>This looks like okay behavior but if you notice the renderTable is still being called when the backspace is pressed. If this was an expensive computation it would be undesirable behavior. </p>
  
  <pre><code>    shinyApp(
    ui = fluidPage(
      selectInput("variable", "Variable:",
                     c("Cylinders" = "cyl",
                       "Transmission" = "am",
                       "Gears" = "gear")),
      tableOutput("data")
    ),
    server = function(input, output) {
      observeEvent(input$variable,{
        cat("Printing: ",input$variable,"\n")
      })
  
      output$data &lt;- renderTable({
        req(input$variable)
        Sys.sleep(2)
        mtcars[, c("mpg", input$variable), drop = FALSE]
      }, rownames = TRUE)
    }
  )
  </code></pre>
  
  <p>My desired behavior: When the backspace is pressed to clear the menu observeEvents and eventReactive are not triggered.</p>