app.R

library(shiny)

zoomPlotOutput <- function(id, ..., zoomLabel = "Zoom") {
  ns <- NS(id)
  tagList(
    plotOutput(ns("plot"), ...),
    downloadButton(ns("zoom"), zoomLabel)
  )
}

zoomPlot <- function(input, output, session, width, height, plotFunc) {
  output$plot <- renderPlot({
    plotFunc()
  })
  url <- session$registerDataObj(session$ns("zoom"), NULL, function(data, req) {
    path <- shiny::plotPNG(plotFunc, width = width, height = height)
    on.exit(unlink(path))
    list(
      status = 200L,
      headers = list("Content-Type" = "image/png"),
      body = readBin(path, raw(), file.size(path))
    )
  })
  output$zoom <- function() { url }
}

ui <- fluidPage(
  zoomPlotOutput("cars")
)

server <- function(input, output, session) {
  callModule(zoomPlot, "cars", 800, 600, function() {
    plot(cars)
  })
}

shinyApp(ui, server)