ggplot not working properly inside eventReactive() in shiny

I wasted hours to find out why my plot is automatically updating itself when I change inputs while it was supposed to wait for the Run button but it simply ignored that step and I ended up finally finding ggplot as the trouble maker!!! This is my minimal code:

library(ggplot2)
library(tidyverse)

varnames <- names(cars)

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      fluidRow(
        column(
          width = 12,

          # Variables Inputs:
          varSelectInput("variables", "Select Input Variables", cars, multiple = TRUE),
          selectizeInput("outvar", "Select Output Variable", choices = varnames, "speed", multiple = F),

          # Run Button
          actionButton(inputId = "run", label = "Run")
        )
      )
    ),

    # Main panel for displaying outputs ----
    mainPanel(
      plotOutput("plot")
    )
  )
)

server <- function(input, output, session) {
  
  df <- reactive({
    cars %>% dplyr::select(!!!input$variables, input$outvar)
  })


  plt <- eventReactive(input$run, {
    
    #Just creating lm formula
    current_formula <- paste0(input$outvar, " ~ ", paste0(input$variables, collapse = " + "))
    current_formula <- as.formula(current_formula)
    #Fitting lm
    fit <- lm(current_formula, data = df())
    pred <- predict(fit, newdata = df())

    #Plotting
    ggplot(df(), aes(df()[, input$outvar], pred)) +
      labs(x = "Observed", y = "Predicted") +
      geom_point() +
      theme_bw()

     #plot(df()[, input$outvar], pred)       #This one works fine!!!!
  })


  output$plot <- renderPlot({
     plt()
  })
}

# Run the application
shinyApp(ui = ui, server = server)

If you run this, you'll notice that ggplot doesn't care anymore about the Run button after the 1st run and it keeps updating as you change the inputs!! However, if you use the simple base plot function (which I put in a comment in the code) there wouldn't be any problems and that works just fine! Sadly I need ggplot in my app because base plot is ugly. Can anyone explain why this happens?

It does seem to me like buggy interaction between ggplot2 and shiny, you could raise this as an issue

I have a kind of work around, which is to wrap with plotly.

library(tidyverse)
library(shiny)
library(plotly)
varnames <- names(cars)

ui <- fluidPage(
  sidebarLayout(
    sidebarPanel(
      fluidRow(
        column(
          width = 12,
          
          # Variables Inputs:
          varSelectInput("variables", "Select Input Variables", cars, multiple = TRUE),
          selectizeInput("outvar", "Select Output Variable", choices = varnames, "speed", multiple = F),
          
          # Run Button
          actionButton(inputId = "run", label = "Run")
        )
      )
    ),
    
    # Main panel for displaying outputs ----
    mainPanel(
      plotlyOutput("plot")
    )
  )
)

server <- function(input, output, session) {
  
  df <- reactive({
    cars %>% dplyr::select(!!!input$variables, input$outvar)
  })

   
  plt <- eventReactive(input$run, {
    
    #Just creating lm formula
    current_formula <- paste0(input$outvar, " ~ ", paste0(input$variables, collapse = " + "))
    current_formula <- as.formula(current_formula)
    #Fitting lm
    fit <- lm(current_formula, data = df())
    pred <- predict(fit, newdata = df())
    
    #Plotting
  p<-  ggplot(df(), aes(df()[, input$outvar], pred)) +
      labs(x = "Observed", y = "Predicted") +
      geom_point() +
      theme_bw() 

  ggplotly(p)
    #plot(df()[, input$outvar], pred)       #This one works fine!!!!
  })
  
  
  output$plot <- renderPlotly({
    plt()
  })
}

# Run the application
shinyApp(ui = ui, server = server)