Design basics

Frames, corners, and order

Each screen is a top-level frame at the exact size of the window.

The frame's top-left corner is the window's origin, and every layer keeps its exact position.

Layers

DoDraw each window as a top-level frame, exactly the size the window should be.

Layers

Don'tDon't use a group or loose layers for a window. Only top-level frames can be picked and converted.

Rounded windows. Give the frame a corner radius and keep Clip content on: the window's corners stay transparent, even under images and glows.
Every corner is exact. Each of the four corners keeps its own radius, so a header with rounded top corners and square bottom corners stays that way.
Layer order is paint order. Backgrounds and glows go at the bottom, then panels, then controls, icons, and text.
The window moves. Dragging any empty part of the generated window moves it; controls keep their clicks.