From 9e042e7accee9c4d41ae53e215e22461592764e2 Mon Sep 17 00:00:00 2001
From: Calixte Denizet <calixte.denizet@gmail.com>
Date: Sat, 24 Feb 2024 22:28:06 +0100
Subject: [PATCH] Avoid to have to wait to zoom after scrolling

Allow to zoom with the wheel once the scrolling is finished.
It's now possible to know that thanks to the scrollend event.

Fixes #17707.
---
 web/app.js | 47 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/web/app.js b/web/app.js
index e9d40b0e2..1cd3447d9 100644
--- a/web/app.js
+++ b/web/app.js
@@ -165,6 +165,9 @@ const PDFViewerApplication = {
   _isCtrlKeyDown: false,
   _nimbusDataPromise: null,
   _caretBrowsing: null,
+  _isScrolling: false,
+  _lastScrollTop: 0,
+  _lastScrollLeft: 0,
 
   // Called once when the document is loaded.
   async initialize(appConfig) {
@@ -683,6 +686,47 @@ const PDFViewerApplication = {
     } else {
       throw new Error("Not implemented: run");
     }
+
+    if (
+      (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) &&
+      !("onscrollend" in document.documentElement)
+    ) {
+      return;
+    }
+
+    const { mainContainer } = appConfig;
+    // Using the values lastScrollTop and lastScrollLeft is a workaround to
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1881974.
+    // TODO: remove them once the bug is fixed.
+    ({ scrollTop: this._lastScrollTop, scrollLeft: this._lastScrollLeft } =
+      mainContainer);
+    const scroll = () => {
+      if (
+        this._lastScrollTop === mainContainer.scrollTop &&
+        this._lastScrollLeft === mainContainer.scrollLeft
+      ) {
+        return;
+      }
+      mainContainer.removeEventListener("scroll", scroll, {
+        passive: true,
+      });
+      this._isScrolling = true;
+      const scrollend = () => {
+        ({ scrollTop: this._lastScrollTop, scrollLeft: this._lastScrollLeft } =
+          mainContainer);
+        this._isScrolling = false;
+        mainContainer.addEventListener("scroll", scroll, {
+          passive: true,
+        });
+        mainContainer.removeEventListener("scrollend", scrollend);
+        mainContainer.removeEventListener("blur", scrollend);
+      };
+      mainContainer.addEventListener("scrollend", scrollend);
+      mainContainer.addEventListener("blur", scrollend);
+    };
+    mainContainer.addEventListener("scroll", scroll, {
+      passive: true,
+    });
   },
 
   get externalServices() {
@@ -2653,6 +2697,7 @@ function webViewerWheel(evt) {
     evt.preventDefault();
     // NOTE: this check must be placed *after* preventDefault.
     if (
+      PDFViewerApplication._isScrolling ||
       zoomDisabledTimeout ||
       document.visibilityState === "hidden" ||
       PDFViewerApplication.overlayManager.active
@@ -2718,8 +2763,6 @@ function webViewerWheel(evt) {
     // left corner is restored. When the mouse wheel is used, the position
     // under the cursor should be restored instead.
     PDFViewerApplication._centerAtPos(previousScale, evt.clientX, evt.clientY);
-  } else {
-    setZoomDisabledTimeout();
   }
 }