From c27dcf2b0331a3efe3da3a957e0056d329342190 Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Wed, 9 Sep 2020 15:07:38 +0200
Subject: [PATCH] Ensure that the `container` div, on `BaseViewer`-instances,
 is absolutely positioned

The `getVisibleElements` helper function currently requires the viewerContainer to be absolutely positioned; possibly fixing this is tracked in issue 11626.

Without `position: absolute;` set, in the CSS, there's a number of things that won't work correctly such as e.g.
 - Determining which pages are currently visible, thus forcing all of them to render on load and increasing resource usage significantly; note https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#allthepages

 - Scrolling pages into view, by using the `BaseViewer.currentPageNumber` setter or similar.

Based on the number of opened issues over the years, the fact that `position: absolute;` is required has shown to be something that users can very easily overlook unless they follow e.g. the `simpleviewer` example to the letter.
Hence, to improve things until such a time that issue 11626 is fixed, we'll now refuse to initialize a `BaseViewer` instance unless the `container` has the required CSS set. (Forcibly setting `position: absolute;` on the viewerContainer element is bound to cause significantly more issues/confusion, hence the current approach of throwing an Error.)
---
 web/base_viewer.js | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/web/base_viewer.js b/web/base_viewer.js
index 9d7ac1d71..36c63cba0 100644
--- a/web/base_viewer.js
+++ b/web/base_viewer.js
@@ -145,14 +145,21 @@ class BaseViewer {
     this.viewer = options.viewer || options.container.firstElementChild;
 
     if (
-      (typeof PDFJSDev === "undefined" ||
-        PDFJSDev.test("!PRODUCTION || GENERIC")) &&
-      !(
-        this.container instanceof HTMLDivElement &&
-        this.viewer instanceof HTMLDivElement
-      )
+      typeof PDFJSDev === "undefined" ||
+      PDFJSDev.test("!PRODUCTION || GENERIC")
     ) {
-      throw new Error("Invalid `container` and/or `viewer` option.");
+      if (
+        !(
+          this.container instanceof HTMLDivElement &&
+          this.viewer instanceof HTMLDivElement
+        )
+      ) {
+        throw new Error("Invalid `container` and/or `viewer` option.");
+      }
+
+      if (getComputedStyle(this.container).position !== "absolute") {
+        throw new Error("The `container` must be absolutely positioned.");
+      }
     }
     this.eventBus = options.eventBus;
     this.linkService = options.linkService || new SimpleLinkService();