Hi Lukas,
Your Gui System is great. At the moment I am creating a small addition to your header with procedural rendered graphics, because I hate to copy/create Bitmaps every time I need a gui in a small test project.
Until now it is only a plain Win98 style, because that's enough for prototypes and small tools.

My problem is:
My window borders are very small bitmaps (6 x 16 pix).
I have seen that you have used bmap_blitt to resize your windows.
The problem is, that my small graphics get "washed out" through the blitting/resizing. I hope you understand my problem
My graphics are exact fitting on eachothers edges, but blurred horizontaly. I don't completly understand the algorithm of the bmap_blitt function. I checked your function and saw that you use the original hight of the middle Titlebar. But it is blurred out as if it would have been scaled in y direction.

My suggestion:
What do you think? Is it usefull to rewrite the scaling function and use the panel element "window". Of course you need a long bitmap with the length of the maximum resolution, but it would be an enorm gain of speed when it comes to resizing.