[Float Request] Add adjustable threads width slider when Discord is small

[Taizunz]

At the moment Discord natively supports adjusting the width of threads that are open in split view, as long as the Discord window isn't lower than 1291px width (in my super-quick testing at least).
I'd love to retain the ability to use this native threads width slider at a much lower window width.

I know I could probably find a solution by looking for a css snippet somewhere, but this specific request also just felt like it was a perfect fit for this particular theme.