1 files changed, 69 insertions, 0 deletions
diff --git a/vnext/src/ui/Header.js b/vnext/src/ui/Header.js
new file mode 100644
index 00000000..48f89360
--- /dev/null
+++ b/vnext/src/ui/Header.js
@@ -0,0 +1,69 @@
+import React, { useEffect, useCallback, useRef } from 'react';
+import ReactDOM from 'react-dom';
+import PropTypes from 'prop-types';
+
+const elClassHidden = 'header--hidden';
+
+const header = document.getElementById('header');
+header.removeChild(document.getElementById('header_wrapper'));
+
+export default function Header({ children }) {
+    let dHeight = useRef(0);
+    let wHeight = useRef(0);
+    let wScrollCurrent = useRef(0);
+    let wScrollBefore = useRef(0);
+    let wScrollDiff = useRef(0);
+
+    useEffect(() => {
+        window.addEventListener('scroll', () => (!window.requestAnimationFrame)
+            ? throttle(250, updateHeader)
+            : window.requestAnimationFrame(updateHeader), false);
+    }, [updateHeader]);
+    let throttle = (delay, fn) => {
+        var last, deferTimer;
+        return function () {
+            var context = this, args = arguments, now = +new Date;
+            if (last && now < last + delay) {
+                clearTimeout(deferTimer);
+                deferTimer = setTimeout(
+                    function () {
+                        last = now;
+                        fn.apply(context, args);
+                    },
+                    delay);
+            } else {
+                last = now;
+                fn.apply(context, args);
+            }
+        };
+    };
+    let updateHeader = useCallback(() => {
+        dHeight.current = document.body.offsetHeight;
+        wHeight.current = window.innerHeight;
+        wScrollCurrent.current = window.pageYOffset;
+        wScrollDiff.current = wScrollBefore.current - wScrollCurrent.current;
+
+        if (wScrollCurrent.current <= 0) {
+            // scrolled to the very top; element sticks to the top
+            header.classList.remove(elClassHidden);
+        } else if (wScrollDiff > 0 && header.classList.contains(elClassHidden)) {
+            // scrolled up; element slides in
+            header.classList.remove(elClassHidden);
+        } else if (wScrollDiff.current < 0) {
+            // scrolled down
+            if (wScrollCurrent.current + wHeight.current >= dHeight.current && header.classList.contains(elClassHidden)) {
+                // scrolled to the very bottom; element slides in
+                header.classList.remove(elClassHidden);
+            } else {
+                // scrolled down; element slides out
+                header.classList.add(elClassHidden);
+            }
+        }
+        wScrollBefore.current = wScrollCurrent.current;
+    }, []);
+    return ReactDOM.createPortal(children, header);
+}
+
+Header.propTypes = {
+    children: PropTypes.node
+};