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

@@ -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();
   }
 }