Hide topbar when mobile orientation in landscape mode (resolve #459)

_javascript/commons/scroll-helper.js

@@ -0,0 +1,36 @@
+/**
+ * A tool for smooth scrolling and topbar switcher
+ */
+const ScrollHelper = (function () {
+  const $body = $("body");
+  const ATTR_TOPBAR_VISIBLE = "topbar-visible";
+  const topbarHeight = $("#topbar-wrapper").outerHeight();
+
+  let scrollUpCount = 0; // the number of times the scroll up was triggered by ToC or anchor
+  let topbarLocked = false;
+  let orientationLocked = false;
+
+  return {
+    hideTopbar: () => $body.attr(ATTR_TOPBAR_VISIBLE, false),
+    showTopbar: () => $body.attr(ATTR_TOPBAR_VISIBLE, true),
+
+    // scroll up
+
+    addScrollUpTask: () => {
+      scrollUpCount += 1;
+      if (!topbarLocked) { topbarLocked = true; }
+    },
+    popScrollUpTask: () => scrollUpCount -= 1,
+    hasScrollUpTask: () => scrollUpCount > 0,
+    topbarLocked: () => topbarLocked === true,
+    unlockTopbar: () => topbarLocked = false,
+    getTopbarHeight: () => topbarHeight,
+
+    // orientation change
+
+    orientationLocked: () => orientationLocked === true,
+    lockOrientation: () => orientationLocked = true,
+    unLockOrientation: () => orientationLocked = false
+  };
+
+}());