It was a Windows-only MFC application from 2002 until this year. The port to Qt 6 is now complete: MFC is entirely gone, and one source tree builds on all three platforms, with #ifdef _WIN32 / #else branches confined to a single platform seam (CbPlatformCompat).

Notes that may be useful to others porting something similar:

Qt 6 forces UNICODE/_UNICODE on anything linking it, while the model is a MultiByte build calling the ANSI Win32 API. Solved by quarantining the Qt code in a static lib that links Qt PRIVATE, and re-stripping the defines on the executable with /UUNICODE /U_UNICODE.
The Win32 named-pipe command server became a QTcpServer on loopback — same JSON protocol on all three platforms.
Two dock bugs in 6.11.1 needed upstream backports (QTBUG-147209 and the group-dissolution crash); patches and write-ups are in the repo under crossplatform/qt-patches/.
GroupedDragging is disabled on all platforms: forming a floating tab group could still be driven into a QMainWindowLayout::animationFinished use-after-free even with both patches applied.
Static Qt on all three, so it ships as a single self-contained binary. Installers for Windows, macOS and Linux: https://github.com/xaljox/ClassBuilder