From 6dfe4a91409bf81fae32dc7b48d97845af67c41e Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Sun, 28 Nov 2021 18:43:30 +0100
Subject: [PATCH] Enforce PAGE-scrolling for *very* large/long documents (bug
 1588435, PR 11263 follow-up)

This patch is essentially a continuation of PR 11263, which tried to improve loading/initialization performance of *very* large/long documents.

Note that browsers, in general, don't handle a huge amount of DOM-elements very well, with really poor (e.g. sluggish scrolling) performance once the number gets "large". Furthermore, at least in Firefox, it seems that DOM-elements towards the bottom of a HTML-page can effectively be ignored; for the PDF.js viewer that means that pages at the end of the document can become impossible to access.

Hence, in order to improve things for these *very* large/long documents, this patch will now enforce usage of the (recently added) PAGE-scrolling mode for these documents. As implemented, this will only happen once the number of pages *exceed* 15000 (which is hopefully rare in practice).
While this might feel a bit jarring to users being *forced* to use PAGE-scrolling, it seems all things considered like a better idea to ensure that the entire document actually remains accessible and with (hopefully) more acceptable performance.

Fixes [bug 1588435](https://bugzilla.mozilla.org/show_bug.cgi?id=1588435), to the extent that doing so is possible since the document contains 25560 pages (and is 197 MB large).
---
 web/base_viewer.js       | 25 +++++++++++++++++++++++--
 web/secondary_toolbar.js | 14 ++++++++++++--
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/web/base_viewer.js b/web/base_viewer.js
index cd78a11a2..3ea105873 100644
--- a/web/base_viewer.js
+++ b/web/base_viewer.js
@@ -54,6 +54,11 @@ import { XfaLayerBuilder } from "./xfa_layer_builder.js";
 
 const DEFAULT_CACHE_SIZE = 10;
 
+const PagesCountLimit = {
+  FORCE_SCROLL_MODE_PAGE: 15000,
+  FORCE_LAZY_PAGE_INIT: 7500,
+};
+
 /**
  * @typedef {Object} PDFViewerOptions
  * @property {HTMLDivElement} container - The container for the viewer element.
@@ -515,6 +520,16 @@ class BaseViewer {
     // Rendering (potentially) depends on this, hence fetching it immediately.
     const optionalContentConfigPromise = pdfDocument.getOptionalContentConfig();
 
+    // Given that browsers don't handle huge amounts of DOM-elements very well,
+    // enforce usage of PAGE-scrolling when loading *very* long/large documents.
+    if (pagesCount > PagesCountLimit.FORCE_SCROLL_MODE_PAGE) {
+      console.warn(
+        "Forcing PAGE-scrolling for performance reasons, given the length of the document."
+      );
+      const mode = (this._scrollMode = ScrollMode.PAGE);
+      this.eventBus.dispatch("scrollmodechanged", { source: this, mode });
+    }
+
     this._pagesCapability.promise.then(() => {
       this.eventBus.dispatch("pagesloaded", {
         source: this,
@@ -618,7 +633,10 @@ class BaseViewer {
 
           // In addition to 'disableAutoFetch' being set, also attempt to reduce
           // resource usage when loading *very* long/large documents.
-          if (pdfDocument.loadingParams.disableAutoFetch || pagesCount > 7500) {
+          if (
+            pdfDocument.loadingParams.disableAutoFetch ||
+            pagesCount > PagesCountLimit.FORCE_LAZY_PAGE_INIT
+          ) {
             // XXX: Printing is semi-broken with auto fetch disabled.
             this._pagesCapability.resolve();
             return;
@@ -1685,6 +1703,9 @@ class BaseViewer {
     if (!isValidScrollMode(mode)) {
       throw new Error(`Invalid scroll mode: ${mode}`);
     }
+    if (this.pagesCount > PagesCountLimit.FORCE_SCROLL_MODE_PAGE) {
+      return; // Disabled for performance reasons.
+    }
     this._previousScrollMode = this._scrollMode;
 
     this._scrollMode = mode;
@@ -1957,4 +1978,4 @@ class BaseViewer {
   }
 }
 
-export { BaseViewer, PDFPageViewBuffer };
+export { BaseViewer, PagesCountLimit, PDFPageViewBuffer };
diff --git a/web/secondary_toolbar.js b/web/secondary_toolbar.js
index fdfb2b638..3b1f33d44 100644
--- a/web/secondary_toolbar.js
+++ b/web/secondary_toolbar.js
@@ -15,6 +15,7 @@
 
 import { SCROLLBAR_PADDING, ScrollMode, SpreadMode } from "./ui_utils.js";
 import { CursorTool } from "./pdf_cursor_tools.js";
+import { PagesCountLimit } from "./base_viewer.js";
 
 /**
  * @typedef {Object} SecondaryToolbarOptions
@@ -235,7 +236,7 @@ class SecondaryToolbar {
   }
 
   _bindScrollModeListener(buttons) {
-    function scrollModeChanged({ mode }) {
+    const scrollModeChanged = ({ mode }) => {
       buttons.scrollPageButton.classList.toggle(
         "toggled",
         mode === ScrollMode.PAGE
@@ -253,13 +254,22 @@ class SecondaryToolbar {
         mode === ScrollMode.WRAPPED
       );
 
+      // Permanently *disable* the Scroll buttons when PAGE-scrolling is being
+      // enforced for *very* long/large documents; please see the `BaseViewer`.
+      const forceScrollModePage =
+        this.pagesCount > PagesCountLimit.FORCE_SCROLL_MODE_PAGE;
+      buttons.scrollPageButton.disabled = forceScrollModePage;
+      buttons.scrollVerticalButton.disabled = forceScrollModePage;
+      buttons.scrollHorizontalButton.disabled = forceScrollModePage;
+      buttons.scrollWrappedButton.disabled = forceScrollModePage;
+
       // Temporarily *disable* the Spread buttons when horizontal scrolling is
       // enabled, since the non-default Spread modes doesn't affect the layout.
       const isScrollModeHorizontal = mode === ScrollMode.HORIZONTAL;
       buttons.spreadNoneButton.disabled = isScrollModeHorizontal;
       buttons.spreadOddButton.disabled = isScrollModeHorizontal;
       buttons.spreadEvenButton.disabled = isScrollModeHorizontal;
-    }
+    };
     this.eventBus._on("scrollmodechanged", scrollModeChanged);
 
     this.eventBus._on("secondarytoolbarreset", evt => {