When zooming the scrollbar can disappear and then no scrollend is triggered

test/integration-boot.mjs

@@ -35,6 +35,7 @@ async function runTests(results) {
       "scripting_spec.mjs",
       "stamp_editor_spec.mjs",
       "text_field_spec.mjs",
+      "viewer_spec.mjs",
     ],
   });
 

test/integration/viewer_spec.mjs

@@ -0,0 +1,58 @@
+/* Copyright 2024 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { closePages, loadAndWait } from "./test_utils.mjs";
+
+describe("PDF viewer", () => {
+  describe("Zoom with the mouse wheel", () => {
+    let pages;
+
+    beforeAll(async () => {
+      pages = await loadAndWait("empty.pdf", ".textLayer .endOfContent", 1000);
+    });
+
+    afterAll(async () => {
+      await closePages(pages);
+    });
+
+    it("must check that we can zoom with the mouse wheel and pressed control key", async () => {
+      await Promise.all(
+        pages.map(async ([browserName, page]) => {
+          if (browserName === "firefox") {
+            // Skip this test for Firefox, as it's not working correctly.
+            // See https://github.com/puppeteer/puppeteer/issues/12093.
+            // TODO: Remove this check once the issue is resolved.
+            return;
+          }
+          await page.keyboard.down("Control");
+          let zoom = 10;
+          const zoomGetter = () =>
+            page.evaluate(
+              () => window.PDFViewerApplication.pdfViewer.currentScale
+            );
+          while (zoom > 0.1) {
+            await page.mouse.wheel({ deltaY: 100 });
+            zoom = await zoomGetter();
+          }
+          while (zoom < 10) {
+            await page.mouse.wheel({ deltaY: -100 });
+            zoom = await zoomGetter();
+          }
+          await page.keyboard.up("Control");
+        })
+      );
+    });
+  });
+});

web/app.js

@@ -2024,8 +2024,9 @@ const PDFViewerApplication = {
     });
     const scroll = (_boundEvents.mainContainerScroll = () => {
       if (
-        this._lastScrollTop === mainContainer.scrollTop &&
+        this._isCtrlKeyDown ||
-        this._lastScrollLeft === mainContainer.scrollLeft
+        (this._lastScrollTop === mainContainer.scrollTop &&
+          this._lastScrollLeft === mainContainer.scrollLeft)
       ) {
         return;
       }