Build a Cross-Platform GUI Text Editor

There is no "text widget" — just a rectangle of pixels the OS lets you draw into. Build a native GUI text editor from first principles in modern C++: a real X11 event loop (with Win32 and Cocoa treated side by side), a software framebuffer, bitmap font rendering, UTF-8 decoding from scratch, a piece-table document model, word-wrap layout, hit testing, selections, damage-driven redraws, the famously weird X11 clipboard protocol, and grouped undo — culminating in a headless EditorCore driven entirely by synthetic events.