In case I can help... Hello, long-time RISC OS user, member of the RO User Group of London. I know some of the people in the RO community and could do introductions, if it'd help. I am trying to learn my way around FreePascal and Lazarus at present because I want to try some things with Ultibo.
Yes, the OS is now open source, since the formation of a new company, RISC OS Developments Ltd. who acquired the license along with the defunct Castle Technologies Ltd. who continued making Acorn computers, and Acorn compatibles, after Acorn closed its workstations division.
(Just to muddy the waters, there are 2 forks of RO. The other fork is _not_ FOSS and is owned by a different company, 3QD:
http://www.riscos.com/legal/terms.htm That company's main business is selling a commercial Acorn computer emulator so that old-time Acorn users can continue to use the OS and apps on top of Windows or macOS. The company now owns its own version of the OS, which a predecessor company bought direct from Acorn. The FOSS fork derives from a separate Acorn licence sold to Pace Technologies, the modem/STB/satellite-receiver company.)
The FOSS fork is called RISC OS Open and is maintained & developed by RISC OS Open Ltd., known as ROOL.
Snag: AIUI, the dev tools to rebuild the OS are _not_ FOSS, and whereas ROOL offer them at a big discount from the previous owners, you still must buy it. Some of RO is written in ARM assembler, but parts are in C and I think they use a compiler formerly called Norcroft C but now just known as the DDE:
https://www.riscosopen.org/content/sales/ddeHowever, that's for rebuilding the OS itself. You don't need it for building apps for the OS. Although it's the official tool, there are multiple alternatives, including GCC:
https://www.riscos.info/index.php/GCC_for_RISC_OSUsing the RO GCCSDK it's also possible to cross-compile for RISC OS on top of GCC's native xNix environments, e.g. ARM Linux or x86 Linux.
RISC OS Open is a native single-core single-threaded 32-bit ARM OS, derived from Pace's work on ARM NCs -- network computers. There is currently no 64-bit version and discussions have only started fairly recently on how they might do one, prompted by ARM releasing 64-bit _only_ ARM chips which can no longer execute 32-bit ARM code.
RISC OS uses cooperative multitasking, and can only pre-empt a text-mode command-line (compare to Windows 2 or 3).
(The original Acorn RISC OS was 26-bit.)
RISC OS' API is like nothing else in the world. It is the only 1980s desktop OS that survives into the modern world. NeXTstep (the basis for Mac OS X) is a couple of years younger.
A decade or so back, there _were_ shim libraries to allow porting of Unix apps (UnixLib), and separately, of X.11 apps (ChoX11), but the RO community were suspicious of these apps and their non-native look and feel. In the end, the developer quit working on RISC OS and emigrated.
So I think any UI integration work would need to be done entirely from scratch. RISC OS has a rich WIMP desktop with all the usual controls, but a radically different look and feel from anything else out there -- because it predates anything else out there. It even predates Windows 2.
So, for instance:
• no menu bars -- the only menus are context menus;
• running an app installs an icon in the "icon bar" at the bottom of the screen. This must be middle-clicked to open a context menu to quit, access the About dialog, etc. By convention, left-clicking it opens the app's first document window;
• no Apply button in dialogs; right-clicking OK applies settings but leaves the dialog open;
• no directory browser or navigation in file-open or file-save dialogs -- Save presents a file icon which the user drags to the destination directory;
• window title bar changes colour and title is suffixed with * to indicate changed contents which should be saved before closing/exiting.
... And many more. Acorn devised and built its own GUI from first principles, starting only a couple of years after the Apple Macintosh was released. Its icon bar informed the Windows 95 taskbar, but little else.
Experience has shown that RISC OS users will refuse apps that don't conform to the rest of the desktop. So, for instance, RISC OS has never, up until now, had a web browser capable of Javascript. (2 are currently in development.) The user base simply refused the port of Firefox from Linux, principally because it did not conform to the RISC OS UI. They'd rather put up with no modern (21st century) web browser than use an alien web-browser with a non-standard look and feel.
I note that, in my experiments on macOS, the Lazarus IDE for FP has a very non-standard UI for a Mac app. Basically, it feels like a Windows app running natively under macOS. I can only say that I strongly suspect that no matter how powerful the tool, the RISC OS user community would refuse such a tool.
Not that I want to be off-putting, as I think this would be a _tremendous_ asset to RISC OS, _especially_ as RISC OS is natively supported on Raspberry Pi, is the fastest OS on the platform, and FPC is also natively supported on bare-metal on RasPi via Ultibo. The possible synergies are tremendous.
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