From 7d1c19f8bd9a7842e56e68791af329bf6871e7ff Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Wed, 21 Jul 2021 09:06:25 +0200
Subject: [PATCH] XFA - Handle `startIndex` correctly in the `Template.$toHTML`
 method (issue 13751)

*Please note:* The PDF document in issue 13751 is *dynamically* created (in e.g. Adobe Reader), with pages added when certain buttons are clicked, hence this patch simply fixes the breaking error and nothing more.

It looks like the current code contains a little bit too much copy-and-paste from the *similar* `index` branch above, since we cannot set the `startIndex` to a negative value. Note how it's being used to initialize the loop-variable, which is then used to lookup values in an Array and accessing the `-1`th element of an Array obviously makes no sense.
---
 src/core/xfa/template.js      | 9 +++++----
 test/pdfs/issue13751.pdf.link | 1 +
 test/test_manifest.json       | 9 +++++++++
 3 files changed, 15 insertions(+), 4 deletions(-)
 create mode 100644 test/pdfs/issue13751.pdf.link

diff --git a/src/core/xfa/template.js b/src/core/xfa/template.js
index 76f23c91c..0670377c1 100644
--- a/src/core/xfa/template.js
+++ b/src/core/xfa/template.js
@@ -5288,13 +5288,14 @@ class Template extends XFAObject {
           } else if (target instanceof ContentArea) {
             const index = contentAreas.findIndex(e => e === target);
             if (index !== -1) {
+              // In the next loop iteration `i` will be incremented, note the
+              // `continue` just below, hence we need to subtract one here.
               i = index - 1;
             } else {
               targetPageArea = target[$getParent]();
-              startIndex =
-                targetPageArea.contentArea.children.findIndex(
-                  e => e === target
-                ) - 1;
+              startIndex = targetPageArea.contentArea.children.findIndex(
+                e => e === target
+              );
             }
           }
           continue;
diff --git a/test/pdfs/issue13751.pdf.link b/test/pdfs/issue13751.pdf.link
new file mode 100644
index 000000000..169f17cdb
--- /dev/null
+++ b/test/pdfs/issue13751.pdf.link
@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/6834770/Oswiadczenie_o_statusie_FATCA_i_CRS-1.pdf
diff --git a/test/test_manifest.json b/test/test_manifest.json
index 5ff77b568..f1c65dddc 100644
--- a/test/test_manifest.json
+++ b/test/test_manifest.json
@@ -1356,6 +1356,15 @@
        "enableXfa": true,
        "type": "eq"
     },
+    {  "id": "issue13751",
+       "file": "pdfs/issue13751.pdf",
+       "md5": "b9960bfa99be3573d9c32a9d55e5b7da",
+       "link": true,
+       "rounds": 1,
+       "lastPage": 1,
+       "enableXfa": true,
+       "type": "eq"
+    },
     {  "id": "issue13756",
        "file": "pdfs/issue13756.pdf",
        "md5": "67040c6df3b5039fcbc81bf63b7c7f98",