Activating Chrome extension over PDF causes page to refresh and jump to title page of PDF

## Bug report form
### Steps to reproduce
1. Open a PDF (either saved locally or hosted elsewhere) in the browser (for example, [this one from MIT Press](https://mitpress.mit.edu/sites/default/files/titles/content/9780262018470_Open_Access_Edition.pdf))
2. Use one of the hyperlinks in the Table of Contents to scroll to a specific page (i.e., the start of Chapter 4)
3. Activate the Hypothesis Chrome extension

### Expected behaviour
One would expect the side bar to pop over the part of the text currently in view.

### Actual behaviour
Instead, the page refreshes and jumps back to the title page of the PDF.

### Browser/system information
Chrome on MacOS, using the h Chrome extension

### Additional details
We can advise users to activate the h extension *before* navigating through the PDF in the meantime. However, this is not in line with how one would expect to use the product. I don't always read something with the intention of annotating - sometimes it just happens that I'm reading and find something interesting, so I switch on the extension to annotate.

See support ticket here: https://hypothesis.zendesk.com/agent/tickets/2001

Start by reproducing the problem in the Chrome extension on macOS using the linked MIT Press PDF: navigate via its table of contents, then activate the extension. Done means the sidebar opens without refreshing the PDF or moving the reader away from the currently viewed page.

Written by the indexing model from the issue text.