Add runtime check of surrounding characters to prevent corruption

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);