From 287776483f01d75a4aef4df6850cf09de233bacd Mon Sep 17 00:00:00 2001
From: Calixte Denizet <calixte.denizet@gmail.com>
Date: Fri, 1 Aug 2025 19:06:14 +0200
Subject: [PATCH] Trigger a fake scrollend event in case it hasn't been
 triggered by the browser (bug 1957680)

This way, it should unblock zooming with ctrl+wheel after scrolling.
---
 web/app.js | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/web/app.js b/web/app.js
index f48643138..69f5ea298 100644
--- a/web/app.js
+++ b/web/app.js
@@ -2227,19 +2227,18 @@ const PDFViewerApplication = {
         mainContainer);
     }
 
+    let scrollendTimeoutID, scrollAbortController;
     const scrollend = () => {
       if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
         ({ scrollTop: this._lastScrollTop, scrollLeft: this._lastScrollLeft } =
           mainContainer);
       }
-
-      this._isScrolling = false;
-      mainContainer.addEventListener("scroll", scroll, {
-        passive: true,
-        signal,
-      });
-      mainContainer.removeEventListener("scrollend", scrollend);
-      mainContainer.removeEventListener("blur", scrollend);
+      clearTimeout(scrollendTimeoutID);
+      if (this._isScrolling) {
+        scrollAbortController.abort();
+        scrollAbortController = null;
+        this._isScrolling = false;
+      }
     };
     const scroll = () => {
       if (this._isCtrlKeyDown) {
@@ -2253,10 +2252,27 @@ const PDFViewerApplication = {
         return;
       }
 
-      mainContainer.removeEventListener("scroll", scroll);
-      this._isScrolling = true;
-      mainContainer.addEventListener("scrollend", scrollend, { signal });
-      mainContainer.addEventListener("blur", scrollend, { signal });
+      if (!this._isScrolling) {
+        scrollAbortController = new AbortController();
+        const abortSignal = AbortSignal.any([
+          scrollAbortController.signal,
+          signal,
+        ]);
+
+        mainContainer.addEventListener("scrollend", scrollend, {
+          signal: abortSignal,
+        });
+        mainContainer.addEventListener("blur", scrollend, {
+          signal: abortSignal,
+        });
+        this._isScrolling = true;
+      }
+      clearTimeout(scrollendTimeoutID);
+      // Why 100 ? Because of:
+      // https://developer.chrome.com/blog/scrollend-a-new-javascript-event
+      // Maybe we could find a better value... ideally the `scrollend` event
+      // should be correctly fired.
+      scrollendTimeoutID = setTimeout(scrollend, 100);
     };
     mainContainer.addEventListener("scroll", scroll, {
       passive: true,