@samhenrigold If you double-click the corner or side of a window, it will extend what you double-clicked as far as it can go.

If you do this while holding Option, it will extend the opposite as well.

So if you do it + Option to a side or top, it will expand a window just horizontally or vertically to its full extent.

If you do it + Option to a corner, you can achieve the old "full screen" window size that existed before the modern full screen experience.