bring back frontend sanity check w/ App.checkBrowserFeatures()

js/common.js

@@ -266,15 +266,18 @@ function fatalError(code, msg, ext_info) {
 			msg = ERRORS[code];
 		}
 
-		let content = "<div><b>Error code:</b> " + code + "</div>" +
+		let content = `<div><b>Error code:</b> ${code} </div>
-			"<p>" + msg + "</p>";
+			<p>${msg}</p>`;
 
 		if (ext_info) {
-			content = content + "<div><b>Additional information:</b></div>" +
+			content = content + `<div><b>Additional information:</b></div>
-				"<textarea style='width: 100%' readonly=\"1\">" +
+				<textarea style='width: 100%' readonly="1">${ext_info}</textarea>`;
-				ext_info + "</textarea>";
 		}
 
+		content += `<div style='text-align : center'>
+			<button dojoType="dijit.form.Button" onclick="window.location.reload()">
+			${__('Try again')}</button></div>`;
+
 		const dialog = new dijit.Dialog({
 			title: "Fatal error",
 			style: "width: 600px",

js/tt-rss.js

@@ -65,6 +65,9 @@ require(["dojo/_base/kernel",
 				constructor: function () {
 					parser.parse();
 
+					if (!this.checkBrowserFeatures())
+						return;
+
 					this.setLoadingProgress(30);
 					this.initHotkeyActions();
 
@@ -89,6 +92,22 @@ require(["dojo/_base/kernel",
 						}
 					});
 				},
+				checkBrowserFeatures: function() {
+					let errorMsg = "";
+
+					['requestIdleCallback', 'MutationObserver'].each(function(wf) {
+						if (! (wf in window)) {
+							errorMsg = `Browser feature check failed: <code>window.${wf}</code> not found.`;
+							throw $break;
+						}
+					});
+
+					if (errorMsg) {
+						fatalError(4, errorMsg, navigator.userAgent);
+					}
+
+					return errorMsg == "";
+				},
 				initSecondStage: function () {
 					this.enableCsrfSupport();