> Win32 was designed around DPI-independence from day one. That's why things like GetSystemMetrics exist. Unfortunately, the app developers chose not to use them.

Integer pixel coordinates are right there in the most fundamental functions:

    HWND WINAPI CreateWindow(LPCTSTR lpClassName,
                             LPCTSTR lpWindowName,
                             DWORD dwStyle,
                             int x,
                             int y,
                             int nWidth,
                             int nHeight,
                             HWND hWndParent,
                             HMENU hMenu,
                             HINSTANCE hInstance,
                             LPVOID lpParam);