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.
Keep the previous way to work for browsers not supporting the
scrollend event.

Fixes #17707.

web/app.js

@@ -165,6 +165,8 @@ const PDFViewerApplication = {
   _isCtrlKeyDown: false,
   _nimbusDataPromise: null,
   _caretBrowsing: null,
+  _isScrolling: false,
+  _isScrollEndSupported: true,
 
   // Called once when the document is loaded.
   async initialize(appConfig) {
@@ -683,6 +685,33 @@ const PDFViewerApplication = {
     } else {
       throw new Error("Not implemented: run");
     }
+
+    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+      this._isScrollEndSupported = "onscrollend" in document.documentElement;
+    }
+    if (
+      (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) ||
+      this._isScrollEndSupported
+    ) {
+      const { mainContainer } = appConfig;
+      mainContainer.addEventListener(
+        "scroll",
+        () => {
+          if (this._isScrolling) {
+            return;
+          }
+          this._isScrolling = true;
+          mainContainer.addEventListener(
+            "scrollend",
+            () => {
+              this._isScrolling = false;
+            },
+            { once: true }
+          );
+        },
+        { passive: true }
+      );
+    }
   },
 
   get externalServices() {
@@ -2653,6 +2682,7 @@ function webViewerWheel(evt) {
     evt.preventDefault();
     // NOTE: this check must be placed *after* preventDefault.
     if (
+      PDFViewerApplication._isScrolling ||
       zoomDisabledTimeout ||
       document.visibilityState === "hidden" ||
       PDFViewerApplication.overlayManager.active
@@ -2718,7 +2748,10 @@ 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 {
+  } else if (
+    (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) &&
+    !PDFViewerApplication._isScrollEndSupported
+  ) {
     setZoomDisabledTimeout();
   }
 }