From e31de7e0a009941669b2eac1644b61b0527245cc Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Tue, 18 Aug 2026 12:17:51 +0200
Subject: [PATCH] Improve the `watchScroll` helper function a little bit

Note how the "scroll" event handler function is attached to the `state`-object, despite that property not being used anywhere in the code-base. This originated in PR 5295, over a decade ago, however the property appears to have been unused from the start; hence let's just remove it now.

Additionally, define the "scroll" event handler function inline and create the `requestAnimationFrame` callback function just once rather than for every new "scroll" event.
---
 web/ui_utils.js | 51 +++++++++++++++++++++++--------------------------
 1 file changed, 24 insertions(+), 27 deletions(-)

diff --git a/web/ui_utils.js b/web/ui_utils.js
index b7c52ab226b33..38dc6feccbfd3 100644
--- a/web/ui_utils.js
+++ b/web/ui_utils.js
@@ -117,43 +117,40 @@ function scrollIntoView(element, spot) {
  * PDF.js friendly one: with scroll debounce and scroll direction.
  */
 function watchScroll(viewAreaElement, callback, abortSignal = undefined) {
-  const debounceScroll = function (evt) {
-    if (rAF) {
-      return;
+  function onRAF() {
+    rAF = null;
+
+    const currentX = viewAreaElement.scrollLeft;
+    const lastX = state.lastX;
+    if (currentX !== lastX) {
+      state.right = currentX > lastX;
     }
-    // schedule an invocation of scroll for next animation frame.
-    rAF = window.requestAnimationFrame(function viewAreaElementScrolled() {
-      rAF = null;
-
-      const currentX = viewAreaElement.scrollLeft;
-      const lastX = state.lastX;
-      if (currentX !== lastX) {
-        state.right = currentX > lastX;
-      }
-      state.lastX = currentX;
-      const currentY = viewAreaElement.scrollTop;
-      const lastY = state.lastY;
-      if (currentY !== lastY) {
-        state.down = currentY > lastY;
-      }
-      state.lastY = currentY;
-      callback(state);
-    });
-  };
+    state.lastX = currentX;
+    const currentY = viewAreaElement.scrollTop;
+    const lastY = state.lastY;
+    if (currentY !== lastY) {
+      state.down = currentY > lastY;
+    }
+    state.lastY = currentY;
+    callback(state);
+  }
 
   const state = {
     right: true,
     down: true,
     lastX: viewAreaElement.scrollLeft,
     lastY: viewAreaElement.scrollTop,
-    _eventHandler: debounceScroll,
   };
 
   let rAF = null;
-  viewAreaElement.addEventListener("scroll", debounceScroll, {
-    useCapture: true,
-    signal: abortSignal,
-  });
+  viewAreaElement.addEventListener(
+    "scroll",
+    () => {
+      // Schedule an invocation of scroll for next animation frame, when needed.
+      rAF ??= window.requestAnimationFrame(onRAF);
+    },
+    { useCapture: true, signal: abortSignal }
+  );
   abortSignal?.addEventListener(
     "abort",
     () => window.cancelAnimationFrame(rAF),