HTML Side-by-side Stops Working After Dismissing a Pop Up on a URL Assignment

Copying from this private issue: https://github.com/hypothesis/support/issues/202

## Bug report form
### Steps to reproduce
1. Go to Incognito mode. This ensures the pop up to show up in Step 3.
2. Access a URL assignment that uses the URL `https://www.edutopia.org/article/scaffolding-complex-topics-middle-school-social-studies`.
Sample assignment: https://hypothesis.instructure.com/courses/676/assignments/9133
3. Scroll in the page with the sidebar open until the pop-up shows up.
4. With the sidebar open, dismiss the sidebar by pressing the ESC key.

Notes:
- If you close the sidebar, close the popup, and open the sidebar, we keep the HTML page pushed over so the Sidebar fits.
- If you dismiss the popup with the Sidebar open (via the escape key) the HTML page underneath expands under the Sidebar.

### Expected behaviour
Side-by-side display will be maintained. Sidebar will not overlap with the webpage.

### Actual behaviour
Side-by-side display is no longer observed. Sidebar overlaps with the webpage.

Sample video:

https://github.com/user-attachments/assets/db4e22df-a4d1-498a-a8cc-0ef76955b69e

### Browser/system information
Chrome 138
Mac OS 15.5

### Additional details
Slack thread: https://hypothes-is.slack.com/archives/C2BLQDKHA/p1751988112662259