Fix task list checkbox toggle to work with YAML front matter (#25184)

Fixes #25160.

`data-source-position` of checkboxes in a task list was incorrect
whenever there was YAML front matter. This would result in issue content
or PR descriptions getting corrupted with random `x` or space characters
when a user checked or unchecked a task.

1 files changed, 8 insertions, 0 deletions

diff --git a/web_src/js/markup/tasklist.js b/web_src/js/markup/tasklist.js
index 0f03837baa..ad1c6964a7 100644
--- a/web_src/js/markup/tasklist.js
+++ b/web_src/js/markup/tasklist.js
@@ -29,6 +29,14 @@ export function initMarkupTasklist() {
 
         const encoder = new TextEncoder();
         const buffer = encoder.encode(oldContent);
+        // Indexes may fall off the ends and return undefined.
+        if (buffer[position - 1] !== '['.codePointAt(0) ||
+          buffer[position] !== ' '.codePointAt(0) && buffer[position] !== 'x'.codePointAt(0) ||
+          buffer[position + 1] !== ']'.codePointAt(0)) {
+          // Position is probably wrong.  Revert and don't allow change.
+          checkbox.checked = !checkbox.checked;
+          throw new Error(`Expected position to be space or x and surrounded by brackets, but it's not: position=${position}`);
+        }
         buffer.set(encoder.encode(checkboxCharacter), position);
         const newContent = new TextDecoder().decode(buffer);