I believe the source code is here: https://github.com/SWI-Prolog/packages-xpce.
It seems to still be actively maintained by the swi-prolog community.
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|XPCE: the SWI-Prolog native GUI library|
XPCE is a toolkit for developing graphical applications in Prolog and other interactive and dynamically typed languages. XPCE follows a rather unique approach to developing GUI applications, which we will try to summarise using the points below.
Prolog-defined methods can receive arguments in native Prolog data, native Prolog data may be associated with XPCE instance-variables and XPCE errors are (selectively) mapped to Prolog exceptions. These features make XPCE a natural extension to your Prolog program.
For starters as well as for more experienced users who want to know how particular tasks are tackled using XPCE/Prolog, there is the XPCE UserGuide. The manual is also available a HTML-tar-archive and can be viewed online.
The reference documentation is available using a hypertext system defined in XPCE/Prolog. This tool exploits the XPCE-class descriptions as well as associated hypertext cards to provide various viewpoints and search mechanisms for browsing the reference material. The manual tools are started using the Prolog command manpce/0:
Finally, the development tools and libraries form a rich set of examples. Just browse through them and then use the Visual Hierarchy Tool to locate the relevant source-code.
On Unix installations, the manpages xpce.1 and xpce-client.1 provide documentation on the command-line options of these commands.