Colored bar appears at the left with multi-line text selections in Chrome

[Rob--W]

Attach (recommended) or Link to PDF file here:

Configuration:

• Web browser and its version: Chromium 92.0.4515.107
• Operating system and its version: ArchLinux
• PDF.js version: master (4cdac66) and
• Is a browser extension: no

Steps to reproduce the problem:

1. Open the demo in Chrome: https://mozilla.github.io/pdf.js/web/viewer.html
2. Select multiple lines

What is the expected behavior?
Only the text should be selected.

What went wrong?
The text is selected, plus a bar appears at the left. This appears to be the result of the dummy <br>. The problem is visible in Chrome, not Firefox.

The issue goes away if I declare a br::selection CSS rule with a transparent color. I'll attach a PR with that patch.