Responsive Design Mode not working correctly

Summary

Responsive Design Mode in the latest Tor Browser is working incorrectly. The content area for the webpage appears bigger than the given value.

Steps to reproduce:

1. Open the Tor Browser and connect to the Tor Network.
2. Browsed a webpage (e.g. https://www.torproject.org/ or https://www.google.com/).
3. Enable Responsive Design Mode. This can be achieved by using keyboard shortcut: Press Ctrl + Shift + M (or Cmd + Opt + M on macOS)

What is the current bug behavior?

The width and height of webpage content area are larger than the configured width and height values (on the top).

What is the expected behavior?

The width and height of webpage content area should be the same as the configured width and height values.

Environment

Effected Tor Browser version:

• Tor Browser 12.5.6 (64-bit) (tested in Ubuntu 22.04.3 LTS and Windows 10 22H2).
• Tor Browser 13.0a6 (64-bit) (tested in Ubuntu 22.04.3 LTS and Windows 10 22H2).

I also tested on Mozilla Firefox 102.15.1esr (Tor Browser 12.5.6 based on) and Mozilla Firefox 115.3.0esr (Tor Browser 13.0a6 based on) to ensure that this bug only occurs in Tor Browser and not the Firefox distribution. As expected, the Responsive Design Mode works correctly in these two Firefox versions. The previous Tor Browser 12.0.7 also works correctly.

Screenshots

Tor Browser 12.5.6

Tor Browser 13.0a6

Mozilla Firefox 102.15.1esr (webpage content area appears correctly)

Mozilla Firefox 115.3.0esr (webpage content area appears correctly)

Tor Browser 12.0.7 (webpage content area appears correctly)