- The borderless window has a frame style for resizing, snapping and its
shadow, with WM_NCCALCSIZE keeping the whole window client. Its edges
and corners resize it; maximised, it is inset by the frame Windows puts
past the screen.
- WM_GETMINMAXINFO holds it to the size it opens at.
- The swap chain's buffers follow the client size (Renderer::resize).
- Frames are drawn from WM_SIZE too, so dragging an edge (Windows' own
message loop) does not stretch the last frame.