The current Window could provide more components and settings to allow easily building windows. I propose some API changes:

• Create WindowTitle (or WindowTitleText, WindowIcon and WindowControls components that can be children of WindowHeader (or WindowTitle)
• Allow title?: React.Node, icon?: React.Node, onClose?: () => void | null, onMinimize?: () => void | null and onMaximize?: () => void | null as shorthands to Window which preconfigure the window title accordingly, and status?: React.Node which renders below the window content.
  • If the events are undefined, the icon isn't displayed, and if they are null the icon is disabled.
• Implement moving and resizing
  • Add x?: number and y?: number for left and top initial positioning respectively
  • Add width?: number and height?: number for the initial window dimensions
  • Implement an onMove: (event: { preventDefault: () => void, { target: { rect: DOMRect } } }) => void, which moves the window but allows the event handler code to prevent that movement (e.g. stick to a frame)
  • Implement an onResize: (event: { preventDefault: () => void, { target: { rect: DOMRect } } }) => void, which resizes the window but allows the event handler code to prevent that resizing (e.g. minimum or maximum sizes)
  • Implement helpers on common that allow users to detect collisions
  • Add a windowResizeWithContent: boolean to the theme that controls if all windows should render their content when being moved or resized, or move as a floating border (Figure 1)
• Implement the variant?: 'toolbar' (or thin) to implement toolbar windows (Figure 1)

Figure 1 (source):
Toolbar window being dragged