`BTerm.draw_box` draws a box but does not fill it.

I'm confused about the use of `BTerm::draw_box`. According to the documentation and underlying code it should draw a filled box. Both, when calling it directly or indirectly via DrawBatch it does not fill the box. This happens both in my own project and the tutorial code.

The attached screenshot shows that.


Also using `draw_box` on an area filled with `DrawBatch.fill_area` removes the color inside the box again.
It seems to me I'm missing something about how rendering works with bracket-lib.