[Bug]: Enabling of "Collapse Tree Style Tabs" option removes resize handle of #sidebar-box

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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 checked with 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.
• [ ] I have checked that this problem is not occurring in the latest version of Firefox ESR.
• [x] This issue is specific to this browser and does not occur in other browsers.

Bug Summary

Enabling of "Collapse Tree Style Tabs" option removes resize handle of #sidebar-box

Reproduction Steps

1. Launch browser with fresh profile
2. Install Sidebery
3. Open Settings and enable "Collapse Tree Style Tabs" option

Expected Behavior

Resize bar handle should be visible and operational

Actual Behavior

Resize bar handle is removed

Screenshots

"Collapse Tree Style Tabs" option is Enabled:

"Collapse Tree Style Tabs" option is Disabled:

Browser Version

11.23.1 (64-bit)

Operating System

Linux 6.8.0-53-generic x86_64, Flatpak

Additional Context

No response