EditPDF: Search comments list stops updating after first invocation

The first time the search comments list is displayed it shows all comments created thus far. However, any comments created after that point will be missing from the list until the page is reloaded (or the editor re-initialised). Other strange behaviour also results.

To reproduce:

1. Create an assignment activity with file upload submissions enabled.
2. As a student, submit a PDF with at least 2 pages.
3. As a teacher, grade the submission and add a couple of comments to page 1.
4. Click the 'Search comments' button and observe that both comment are listed.
5. Close the dialogue by either selecting a comment or clicking the close button.
6. Add a third comment to the same page.
7. Invoke the 'Search comments' tool again and observe that the third comment is missing.
8. Close the dialogue again (by either method) and then navigate to page 2 using the page navigation.
9. Use the 'Search comments' tool to select a comment.
10. Observe that you are returned to page 1, but it now appears to contain only 2 comments.
11. Use the page navigation to navigate to page 2 and then back to page 1.
12. Observe that page 1 now appears to contain 3 comments again.
13. Attempting to select a comment from the 'Search comments' list again at this point will often result in failure to locate and focus the comment node.