There are a thin bar below the windows

[rinmyo]

Looks very strange. idk if it is a bug.

[ireggiani]

If you are referring to that black bar at the bottom, it might have been done that way to achieve the rounded borders Mojave has on OSX. If you look at it, the height of the bar is just enough for the radius of the rounded borders on the left and right.

The bar is not noticeable on native applications, as it blends into the rest of the window, at least.

I'd prefer it to not be there; but it might not be possible without losing part of the style.

[y-usuzumi]

Some windows have the resizing indicator at the bottom right. Looks really ugly with the bar.

[a-parhom]

I recommend using ShapeCorners KWin effect with SierraBreeze window decoration.