1 files changed, 65 insertions, 0 deletions
diff --git a/shared/utils/src/try-scroll.ts b/shared/utils/src/try-scroll.ts
new file mode 100644
index 0000000..1e6b0d2
--- /dev/null
+++ b/shared/utils/src/try-scroll.ts
@@ -0,0 +1,65 @@
+import type { Logger } from '@amp/web-apps-logger';
+export interface ScrollableElement {
+    scrollTop: number;
+    scrollHeight: number;
+    offsetHeight: number;
+}
+
+// Global is okay here as this only runs in the browser
+let nextTry: number | null = null;
+
+export function tryScroll(
+    log: Logger,
+    getScrollablePageElement: Function,
+    scrollY: number,
+): void {
+    let tries = 0;
+
+    if (nextTry !== null) {
+        window.cancelAnimationFrame(nextTry);
+    }
+
+    nextTry = window.requestAnimationFrame(function doNextTry() {
+        // At 16ms per frame, this is 1600ms
+        // See: https://github.com/DockYard/ember-router-scroll/blob/2f17728f/addon/services/router-scroll.js#L56
+        if (++tries >= 100) {
+            log.warn("wasn't able to restore scroll within 100 frames");
+            nextTry = null;
+            return;
+        }
+
+        let element = getScrollablePageElement();
+        if (!element) {
+            log.warn(
+                'could not restore scroll: the scrollable element is missing',
+            );
+            return;
+        }
+        const { scrollHeight, offsetHeight } = element;
+
+        // Only scroll once we're able to get a full screen of content when
+        // scrollTop is set to scrollY
+        //
+        // +16 is a bit of a fudge factor to count for imperfections in
+        // features like lazy loading. If the scroll position to restore is
+        // the very bottom of the page, then scrollY + offsetHeight must be
+        // exactly scrollHeight. But if lazy loading components (for example)
+        // cause the page to grow by a few pixels, then this will never hold.
+        // Thus, we fudge by a few pixels to be more forgiving in this scenario.
+        const canScroll = scrollY + offsetHeight <= scrollHeight + 16;
+
+        if (!canScroll) {
+            log.info('page is not tall enough for scroll yet', {
+                scrollHeight,
+                offsetHeight,
+            });
+
+            nextTry = window.requestAnimationFrame(doNextTry);
+            return;
+        }
+
+        element.scrollTop = scrollY;
+        log.info('scroll restored to', scrollY);
+        nextTry = null;
+    });
+}