works

.gitignore

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+.Rproj.user
+.Rhistory
+.RData
+.Ruserdata

app.R

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+# https://html2canvas.hertzen.com
+# Limitations: browser support see, some CSS properties (which I think is the reason why leaflet shows up as just gray), more info at https://html2canvas.hertzen.com/documentation
+# Also, images that have a large size (I haven't tested, but it seems when the canvas blobk is over 1 or 2 GB for me) it fails due to memory issues.
+
+library(shiny)
+
+source("capture.R")
+
+ui <- fluidPage(
+  textInput("text", "Selector to capture", "#plot"),
+  textInput("name", "name", "image.png"),
+  actionButton("screenshot", "Screenshot"),
+  plotOutput("plot"),
+  plotly::plotlyOutput("plotly"),
+  numericInput("num", "Number", 10)
+)
+
+server <- function(input, output, session) {
+  output$plot <- renderPlot({
+    plot(seq(input$num))
+  })
+
+  output$plotly <- plotly::renderPlotly({
+    plotly::plot_ly(iris, x = ~Sepal.Width, y = ~Sepal.Length)
+  })
+
+  observeEvent(input$screenshot, {
+    docapture(selector = input$text, filename = input$name)
+  })
+}
+
+shinyApp(ui, server)

capture.R

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+
+docapture <- function(id = "", selector = "body", filename = "screenshot.png") {
+  setupcapture()
+
+  if (nzchar(id)) {
+    selector <- paste0("#", id)
+  }
+
+  session <- getSession()
+  session$sendCustomMessage("screenshot", list(selector = selector, filename = filename))
+}
+
+getSession <- function() {
+  session <- shiny::getDefaultReactiveDomain()
+
+  if (is.null(session)) {
+    stop("Could not find a Shiny session.")
+  }
+
+  session
+}
+
+setupcapture <- function() {
+  session <- getSession()
+
+  if (!is.null(attr(session, "shinyshot_attached"))) {
+    return()
+  }
+  attr(session, "shinyshot_attached") <- TRUE
+
+  shiny::addResourcePath("shinyscreenshot", "~/../R/shinyscreenshot/www/")
+
+  shiny::insertUI("head", "beforeEnd", {
+    shiny::singleton(shiny::tags$head(
+      shiny::tags$script(src = "shinyscreenshot/saveFile.js"),
+      shiny::tags$script(src = "shinyscreenshot/html2canvas.min.js")
+    ))
+  }, immediate = TRUE)
+}

shinyscreenshot.Rproj

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+Version: 1.0
+
+RestoreWorkspace: Default
+SaveWorkspace: Default
+AlwaysSaveHistory: Default
+
+EnableCodeIndexing: Yes
+UseSpacesForTab: Yes
+NumSpacesForTab: 2
+Encoding: UTF-8
+
+RnwWeave: Sweave
+LaTeX: pdfLaTeX
+
+AutoAppendNewline: Yes
+StripTrailingWhitespace: Yes

www/saveFile.js

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+Shiny.addCustomMessageHandler('screenshot', function(params) {
+    html2canvas($(params.selector)[0]).then(function(canvas) {
+        img = canvas.toDataURL();
+        saveFile(img, params.filename);
+    });
+});
+
+// heavily borrowed from plotly.js
+var saveFile = function(url, filename) {
+    var saveLink = document.createElement('a');
+    var canUseSaveLink = 'download' in saveLink;
+    var isSafari = /Version\/[\d\.]+.*Safari/.test(navigator.userAgent);
+    // IE <10 is explicitly unsupported
+    if (typeof navigator !== 'undefined' && /MSIE [1-9]\./.test(navigator.userAgent)) {
+        alert('Download not supported in IE < 10');
+    }
+
+    // First try a.download, then web filesystem, then object URLs
+    if (isSafari) {
+        // Safari doesn't allow downloading of blob urls
+        document.location.href = 'data:application/octet-stream' + url.slice(url.search(/[,;]/));
+        return;
+    }
+
+    if (canUseSaveLink) {
+        saveLink.href = url;
+        saveLink.download = filename;
+        document.body.appendChild(saveLink);
+        saveLink.click();
+        document.body.removeChild(saveLink);
+        return;
+    }
+
+    // IE 10+ (native saveAs)
+    if(typeof navigator !== 'undefined' && navigator.msSaveBlob) {
+        // At this point we are only dealing with a SVG encoded as
+        // a data URL (since IE only supports SVG)
+        var encoded = url.split(/^data:image\/svg\+xml,/)[1];
+        var svg = decodeURIComponent(encoded);
+        navigator.msSaveBlob(new Blob([svg]), filename);
+        return;
+    }
+
+    alert('Download error');
+};