Shiny - The grey bar is not follow the size of the plots bigger than the window size

app.R

library(shiny)
library(shinyjs)
library(DT)
data("cars")

ui <- tagList(
  useShinyjs(),
  navbarPage("OSD",
             tabPanel("Multi",
                      sidebarLayout(
                        sidebarPanel(
                          selectInput("select_size", "Select plot number", choices=c("1","2","3"))
                        ),
                        mainPanel(
                          uiOutput("mplots")
                        )
                      )
             )  
  )
)


server <- function(input, output, session) {
  
  output$plot1 <- renderPlot({
    plot(cars)
  }, height = 300, width = 300)
  
  output$plot2 <- renderPlot({
    plot(cars)
  }, height = 500, width = 500)
  
  output$plot3 <- renderPlot({
    plot(cars)
  }, height = 800, width = 1200)
  
  output$mplots <- renderUI({
    if(input$select_size=="1"){
      list(
        tags$h3("This is ok"),
        plotOutput("plot1")
      )
    }else if(input$select_size=="2"){
      list(
        tags$h3("This is also ok"),
        plotOutput("plot2")
      )
    }else if(input$select_size=="3"){
      list(
        tags$h3("This is not ok"),
        tags$p("Imagine that this is a heatmap. They are big, from that follows the need to generate the plot in this way"),
        plotOutput("plot3")
      )
     
    }
  })
}

shinyApp(ui, server)