refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/topbar-switcher.js

@@ -0,0 +1,93 @@
+/**
+ * Hide Header on scroll down
+ */
+import ScrollHelper from './utils/scroll-helper';
+
+const $searchInput = $('#search-input');
+const delta = ScrollHelper.getTopbarHeight();
+
+let didScroll;
+let lastScrollTop = 0;
+
+function hasScrolled() {
+  let st = $(window).scrollTop();
+
+  /* Make sure they scroll more than delta */
+  if (Math.abs(lastScrollTop - st) <= delta) {
+    return;
+  }
+
+  if (st > lastScrollTop) {
+    /* Scroll down */
+    ScrollHelper.hideTopbar();
+
+    if ($searchInput.is(':focus')) {
+      $searchInput.trigger('blur'); /* remove focus */
+    }
+  } else {
+    /* Scroll up */
+
+    // has not yet scrolled to the bottom of the screen, that is, there is still space for scrolling
+    if (st + $(window).height() < $(document).height()) {
+      if (ScrollHelper.hasScrollUpTask()) {
+        return;
+      }
+
+      if (ScrollHelper.topbarLocked()) {
+        // avoid redundant scroll up event from smooth scrolling
+        ScrollHelper.unlockTopbar();
+      } else {
+        if (ScrollHelper.orientationLocked()) {
+          // avoid device auto scroll up on orientation change
+          ScrollHelper.unLockOrientation();
+        } else {
+          ScrollHelper.showTopbar();
+        }
+      }
+    }
+  }
+
+  lastScrollTop = st;
+} // hasScrolled()
+
+function handleLandscape() {
+  if ($(window).scrollTop() === 0) {
+    return;
+  }
+  ScrollHelper.lockOrientation();
+  ScrollHelper.hideTopbar();
+}
+
+export function switchTopbar() {
+  const orientation = screen.orientation;
+  if (orientation) {
+    orientation.onchange = () => {
+      const type = orientation.type;
+      if (type === 'landscape-primary' || type === 'landscape-secondary') {
+        handleLandscape();
+      }
+    };
+  } else {
+    // for the browsers that not support `window.screen.orientation` API
+    $(window).on('orientationchange', () => {
+      if ($(window).width() < $(window).height()) {
+        // before rotating, it is still in portrait mode.
+        handleLandscape();
+      }
+    });
+  }
+
+  $(window).on('scroll', () => {
+    if (didScroll) {
+      return;
+    }
+    didScroll = true;
+  });
+
+  setInterval(() => {
+    if (didScroll) {
+      hasScrolled();
+      didScroll = false;
+    }
+  }, 250);
+}