[Bug]: Sites loaded in the sidebar in a container and with a mobile user-agent reload every time they gain focus

[yawns]

Pre-Submission Checklist

• [X] I have checked the existing/closed issues to ensure this issue has not already been reported.
• [X] I am using the latest supported version of the browser.
• [X] I have tried disabling all browser extensions/add-ons to rule out interference from these.
• [X] I have tried clearing the browser cache and cookies.
• [X] I have tried to create a new profile and the issue still occurs.
• [X] I have provided detailed step-by-step instructions on how to reproduce the issue.
• [X] I have included relevant screenshots or console outputs.
• [X] I have checked this problem is not ocurring in the latest version of Firefox ESR.
• [X] This issue is specific to this browser and does not occur in other browsers.

Bug Summary

This is a pretty specific scenario: If you load a site in the sidebar, and set it to use both a container and a mobile user-agent, it will reload every time it regains focus. This causes problems with things like music playback or videos, because it stops the media from playing.

This only happens if both the mobile UA is set, and it's in a container. A mobile UA outside of a container has no issue, and a desktop UA in a container is also fine.

This occurs on every website. Sleeping tabs and "unload when hiding" are both disabled.

In the attached video, I'm using a new profile with no addons. Initially I have the Youtube panel in a container and using a mobile user-agent. You can see that it reloads as I click to another panel then back (and the video stops playing).

For whatever reason the menus in the browser aren't being captured in the recording, but at 40 seconds in I change the panel to use the Desktop UA. After the change, you can see the panel no longer reloads when I switch away and then back to it.

Reproduction Steps

1. Go to youtube.com (or any site where a reload would be disruptive).
2. Right click and "Add page to web panel."
3. Select a container under, "Use the following container."
4. Check the box to "Use mobile user agent."
5. Open the sidebar panel you just created.
6. Play a video.
7. Open any other panel -- the video will keep playing in the background.
8. Click back to the youtube panel -- the panel will reload to the YT front page.

Expected Behavior

The panel shouldn't reload unless the user initiates it.

Actual Behavior

The panel reloads on its own each time it regains focus (or is moved to the top -- not sure what's actually happening to make these panels work).

Screenshots

https://github.com/Floorp-Projects/Floorp/assets/2416785/01e0a1e1-a7b4-4068-a008-fbf7b297239c

Browser Version

11.11.2

Operating System

MacOS Sonoma 14.4

Additional Context

No response