1 files changed, 66 insertions, 0 deletions
diff --git a/gfx/layers/apz/test/mochitest/browser_test_background_tab_scroll.js b/gfx/layers/apz/test/mochitest/browser_test_background_tab_scroll.js
new file mode 100644
index 0000000000..4ce8200199
--- /dev/null
+++ b/gfx/layers/apz/test/mochitest/browser_test_background_tab_scroll.js
@@ -0,0 +1,66 @@
+add_task(async function test_main() {
+  // Load page in the background. This will cause the first-paint of the
+  // tab (which has ScrollPositionUpdate instances) to get sent to the
+  // compositor, but the parent process RefLayer won't be pointing to this
+  // tab so APZ never sees the ScrollPositionUpdate instances.
+
+  let url =
+    "http://mochi.test:8888/browser/gfx/layers/apz/test/mochitest/helper_background_tab_scroll.html#scrolltarget";
+  let backgroundTab = BrowserTestUtils.addTab(gBrowser, url);
+  let browser = backgroundTab.linkedBrowser;
+  await BrowserTestUtils.browserLoaded(browser, false, url);
+  dump("Done loading background tab\n");
+
+  // Switch to the foreground, to let the APZ tree get built.
+  await BrowserTestUtils.switchTab(gBrowser, backgroundTab);
+  dump("Switched background tab to foreground\n");
+
+  // Verify main-thread scroll position is where we expect
+  let scrollPos = await ContentTask.spawn(browser, null, function () {
+    return content.window.scrollY;
+  });
+  is(scrollPos, 5000, "Expected background tab to be at scroll pos 5000");
+
+  // Trigger an APZ-side scroll via native wheel event, followed by some code
+  // to ensure APZ's repaint requests to arrive at the main-thread. If
+  // things are working properly, the main thread will accept the repaint
+  // requests and update the main-thread scroll position. If the APZ side
+  // is sending incorrect scroll generations in the repaint request, then
+  // the main thread will fail to clear the main-thread scroll origin (which
+  // was set by the scroll to the #scrolltarget anchor), and so will not
+  // accept APZ's scroll position updates.
+  let contentScrollFunction = async function () {
+    await content.window.wrappedJSObject.promiseNativeWheelAndWaitForWheelEvent(
+      content.window,
+      100,
+      100,
+      0,
+      200
+    );
+
+    // Advance some/all frames of the APZ wheel animation
+    let utils = content.window.SpecialPowers.getDOMWindowUtils(content.window);
+    for (var i = 0; i < 10; i++) {
+      utils.advanceTimeAndRefresh(16);
+    }
+    utils.restoreNormalRefresh();
+    // Flush pending APZ repaints, then read the main-thread scroll
+    // position
+    await content.window.wrappedJSObject.promiseOnlyApzControllerFlushed(
+      content.window
+    );
+    return content.window.scrollY;
+  };
+  scrollPos = await ContentTask.spawn(browser, null, contentScrollFunction);
+
+  // Verify main-thread scroll position has changed
+  ok(
+    scrollPos < 5000,
+    `Expected background tab to have scrolled up, is at ${scrollPos}`
+  );
+
+  // Cleanup
+  let tabClosed = BrowserTestUtils.waitForTabClosing(backgroundTab);
+  BrowserTestUtils.removeTab(backgroundTab);
+  await tabClosed;
+});