scroll first scrollable elment

src/content/scrolls.js

@@ -1,47 +1,112 @@
 const SCROLL_DELTA_X = 48;
 const SCROLL_DELTA_Y = 48;
 
-const scrollVertically = (page, count) => {
-  let x = page.scrollX;
-  let y = page.scrollY + SCROLL_DELTA_X * count;
-  page.scrollTo(x, y);
+const isVisible = (win, element) => {
+  let rect = element.getBoundingClientRect();
+  if (rect.width === 0 || rect.height === 0) {
+    return false;
+  }
+  if (rect.right < 0 && rect.bottom < 0) {
+    return false;
+  }
+  if (win.innerWidth < rect.left && win.innerHeight < rect.top) {
+    return false;
+  }
+
+  let { display, visibility } = win.getComputedStyle(element);
+  if (display === 'none' || visibility === 'hidden') {
+    return false;
+  }
+  return true;
 };
 
-const scrollHorizonally = (page, count) => {
-  let x = page.scrollX + SCROLL_DELTA_Y * count;
-  let y = page.scrollY;
-  page.scrollTo(x, y);
+const isScrollable = (win, element) => {
+  let { overflowX, overflowY } = win.getComputedStyle(element);
+  if (element.tagName !== 'HTML' &&
+    overflowX !== 'scroll' && overflowX !== 'auto' &&
+    overflowY !== 'scroll' && overflowY !== 'auto') {
+    return false;
+  }
+  return element.scrollWidth > element.clientWidth ||
+    element.scrollHeight > element.clientHeight;
 };
 
-const scrollPages = (page, count) => {
-  let height = page.innerHeight;
-  let x = page.scrollX;
-  let y = page.scrollY + height * count;
-  page.scrollTo(x, y);
+// Find a visiable and scrollable element by depth-first search.  Currently
+// this method is called by each scrolling, and the returned value of this
+// method is not cached.  That does not cause performance issue because in the
+// most pages, the window is root element i,e, documentElement.
+const findScrollable = (win, element) => {
+  if (isScrollable(win, element)) {
+    return element;
+  }
+
+  let children = Array.prototype
+    .filter.call(element.children, e => isVisible(win, e));
+  for (let child of children) {
+    let scrollable = findScrollable(win, child);
+    if (scrollable) {
+      return scrollable;
+    }
+  }
+  return null;
 };
 
-const scrollTop = (page) => {
-  let x = page.scrollX;
+const scrollTarget = (win) => {
+  let target = findScrollable(win, win.document.documentElement);
+  if (target) {
+    return target;
+  }
+  return win.document.documentElement;
+};
+
+const scrollVertically = (win, count) => {
+  let target = scrollTarget(win);
+  let x = target.scrollLeft;
+  let y = target.scrollTop + SCROLL_DELTA_Y * count;
+  target.scrollTo(x, y);
+};
+
+const scrollHorizonally = (win, count) => {
+  let target = scrollTarget(win);
+  let x = target.scrollLeft + SCROLL_DELTA_X * count;
+  let y = target.scrollTop;
+  target.scrollTo(x, y);
+};
+
+const scrollPages = (win, count) => {
+  let target = scrollTarget(win);
+  let height = target.innerHeight;
+  let x = target.scrollLeft;
+  let y = target.scrollLeft + height * count;
+  target.scrollTo(x, y);
+};
+
+const scrollTop = (win) => {
+  let target = scrollTarget(win);
+  let x = target.scrollLeft;
   let y = 0;
-  page.scrollTo(x, y);
+  target.scrollTo(x, y);
 };
 
-const scrollBottom = (page) => {
-  let x = page.scrollX;
-  let y = page.scrollMaxY;
-  page.scrollTo(x, y);
+const scrollBottom = (win) => {
+  let target = scrollTarget(win);
+  let x = target.scrollLeft;
+  let y = target.scrollHeight;
+  target.scrollTo(x, y);
 };
 
-const scrollHome = (page) => {
+const scrollHome = (win) => {
+  let target = scrollTarget(win);
   let x = 0;
-  let y = page.scrollY;
-  page.scrollTo(x, y);
+  let y = target.scrollLeft;
+  target.scrollTo(x, y);
 };
 
-const scrollEnd = (page) => {
-  let x = page.scrollMaxX;
-  let y = page.scrollY;
-  page.scrollTo(x, y);
+const scrollEnd = (win) => {
+  let target = scrollTarget(win);
+  let x = target.scrollWidth;
+  let y = target.scrollLeft;
+  target.scrollTo(x, y);
 };
 
 export {