Drop ifvisible as it was only used for Prefetch and that feature was removed in v2.17

.eslintrc.js

@@ -27,8 +27,6 @@ module.exports = {
 		'SquireUI': "readonly",
 		// node_modules/knockout but dev/External/ko.js is used
 		'ko': "readonly",
-		// dev/External/ifvisible.js
-		'ifvisible': "readonly",
 		// vendors/routes/
 		'hasher': "readonly",
 		'Crossroads': "readonly",

README.md

@@ -108,7 +108,6 @@ The result is faster and smaller download code (good for mobile networks).
 * boot.js without webpack overhead
 * Modified Jua.js to be without jQuery
 * Replaced Autolinker with simple https/email detection
-* Replaced ifvisible.js with simple drop-in replacement
 * Replaced momentToNode with proper HTML5 `<time>`
 * Replaced resize listeners with ResizeObserver
 * Replaced bootstrap.js with native drop-in replacement
@@ -119,6 +118,7 @@ The result is faster and smaller download code (good for mobile networks).
 * Replaced simplestatemanager with CSS @media
 * Replaced inputosaurus with own code
 * Replaced keymaster with own shortcuts handler
+* Removed ifvisible.js
 * Removed pikaday
 * Removed underscore
 * Removed polyfills

dev/External/ifvisible.js

@@ -1,40 +0,0 @@
-(doc => {
-	let idle = 'idle',
-		visible = 'visible',
-		status = visible,
-		timer = 0,
-		wakeUp = () => {
-			clearTimeout(timer);
-			status = visible;
-			timer = setTimeout(() => {
-				if (status === visible) {
-					status = idle;
-					dispatchEvent(new CustomEvent(idle));
-				}
-			}, 10000);
-		},
-		init = () => {
-			init = () => 0;
-			// Safari
-			addEventListener('pagehide', () => status = 'hidden');
-			// Else
-			doc.addEventListener('visibilitychange', () => {
-				status = doc.visibilityState;
-				doc.hidden || wakeUp();
-			});
-			wakeUp();
-			['mousemove','keyup','touchstart'].forEach(t => doc.addEventListener(t, wakeUp));
-			['scroll','pageshow'].forEach(t => addEventListener(t, wakeUp));
-		};
-
-	this.ifvisible = {
-		idle: callback => {
-			init();
-			addEventListener(idle, callback);
-		},
-		now: () => {
-			init();
-			return status === visible;
-		}
-	};
-})(document);

tasks/config.js

@@ -60,7 +60,6 @@ config.paths.js = {
 		name: 'libs.js',
 		src: [
 			'dev/prototype.js',
-			'dev/External/ifvisible.js',
 			'dev/dragdropgecko.js',
 			'dev/shortcuts.js',
 			'vendors/routes/hasher.js',