refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/utils/scroll-helper.js

@@ -0,0 +1,64 @@
+/**
+ * A tool for smooth scrolling and topbar switcher
+ */
+
+const ATTR_TOPBAR_VISIBLE = 'data-topbar-visible';
+const $body = $('body');
+const $topbarWrapper = $('#topbar-wrapper');
+
+export default class ScrollHelper {
+  static scrollUpCount = 0; // the number of times the scroll up was triggered by ToC or anchor
+  static topbarIsLocked = false;
+  static orientationIsLocked = false;
+
+  static hideTopbar() {
+    $body.attr(ATTR_TOPBAR_VISIBLE, 'false');
+  }
+
+  static showTopbar() {
+    $body.attr(ATTR_TOPBAR_VISIBLE, 'true');
+  }
+
+  // scroll up
+
+  static addScrollUpTask() {
+    ScrollHelper.scrollUpCount += 1;
+    if (!ScrollHelper.topbarIsLocked) {
+      ScrollHelper.topbarIsLocked = true;
+    }
+  }
+
+  static popScrollUpTask() {
+    ScrollHelper.scrollUpCount -= 1;
+  }
+
+  static hasScrollUpTask() {
+    return ScrollHelper.scrollUpCount > 0;
+  }
+
+  static topbarLocked() {
+    return ScrollHelper.topbarIsLocked === true;
+  }
+
+  static unlockTopbar() {
+    ScrollHelper.topbarIsLocked = false;
+  }
+
+  static getTopbarHeight() {
+    return $topbarWrapper.outerHeight();
+  }
+
+  // orientation change
+
+  static orientationLocked() {
+    return ScrollHelper.orientationIsLocked === true;
+  }
+
+  static lockOrientation() {
+    ScrollHelper.orientationIsLocked = true;
+  }
+
+  static unLockOrientation() {
+    ScrollHelper.orientationIsLocked = false;
+  }
+}