Add download screenshot button

app/ui.js

@@ -766,12 +766,21 @@ const UI = {
         });
     },
 
+    attachDownloadScreenshotButton() {
+        document.getElementById('noVNC_download_screenshot').addEventListener('click', function() {
+            let link = document.createElement('a');
+            link.download = 'screenshot.png';
+            link.href = UI.rfb.canvas.toDataURL("image/png");
+            link.click();
+        });
+    },
+
     updateInteractionStackUI() {
-        document.getElementById('noVNC_click_stack').innerHTML = "";
+        document.getElementById('noVNC_click_stack').innerHTML = '';
         for (var i = 0; i < UI.canvasInteractionEvents.length; i++) {
             let e = UI.canvasInteractionEvents[i];
             let el = document.createElement('li');
-            el.innerText = e.name + " at (" + e.x + ", " + e.y + ")";
+            el.innerText = e.name + ' at (' + e.x + ', ' + e.y + ')';
             document.getElementById('noVNC_click_stack').append(el);
         }
     },
@@ -1108,6 +1117,7 @@ const UI = {
 
         UI.trackMouse();
         UI.trackClicks();
+        UI.attachDownloadScreenshotButton();
 
         UI.updateViewOnly(); // requires UI.rfb
     },

vnc.html

@@ -16,7 +16,7 @@
     <title>noVNC</title>
 
     <meta charset="utf-8">
-    
+
     <!-- Always force latest IE rendering engine (even in intranet) & Chrome Frame
                 Remove this if you use the .htaccess -->
     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
@@ -338,6 +338,10 @@
                 (0, 0)
             </div>
         </div>
+        <div class="noVNC_info_item">
+            <span class="noVNC_info_item_label">Screenshot</span>
+            <button id="noVNC_download_screenshot">Download screenshot</button>
+        </div>
         <div class="noVNC_info_item">
             <span class="noVNC_info_item_label">Interaction stack</span>
             <button id="noVNC_click_stack_copy">Copy stack to clipboard</button>