Please note the build is terminated after 2 days and 2 hours (the maximum that Copr allows), so I don't have the full logs, but one issue is related to Python 3.14:

Compiling xpra/net/lz4/lz4.pyx because it changed.
Traceback (most recent call last):
  File "/usr/lib64/python3.14/multiprocessing/forkserver.py", line 335, in main
    code = _serve_one(child_r, fds,
                      unused_fds,
                      old_handlers)
'nvcc --version' failed with return code 127
stderr: None
updated ./xpra/build_info.py with:
* BUILD_BIT            = 64bit
* BUILD_CPU            = Intel(R) Xeon(R) Platinum 8488C
* BUILD_DATE           = 2025-01-22
* BUILD_MACHINE        = x86_64
* BUILD_OS             = linux
* BUILD_TIME           = 00:00
* COMPILER_VERSION     = gcc (GCC) 15.0.1 20250225 (Red Hat 15.0.1-0)
* CYTHON_VERSION       = 3.0.11
* LINKER_VERSION       = GNU ld version 2.44-3.fc43
* PYTHON_VERSION       = 3.14.0
* lib_gobjectintrospection10 = 1.82.0
* lib_gtk30            = 3.24.43
* lib_gtkx1130         = 3.24.43
* lib_py3cairo         = 1.25.1
* lib_pygobject30      = 3.51.0
* lib_python3          = 3.14
* lib_vpx              = 1.15.0
* lib_x11              = 1.8.11
* lib_xcomposite       = 0.4.6
* lib_xdamage          = 1.1.6
* lib_xext             = 1.3.6
* lib_xfixes           = 6.0.1
* lib_xkbfile          = 1.1.3
* lib_xrandr           = 1.5.4
* lib_xtst             = 1.2.5
  File "/usr/lib64/python3.14/multiprocessing/forkserver.py", line 375, in _serve_one
    code = spawn._main(child_r, parent_sentinel)
  File "/usr/lib64/python3.14/multiprocessing/spawn.py", line 131, in _main
    prepare(preparation_data)
    ~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/spawn.py", line 246, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/spawn.py", line 297, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
                                  run_name="__mp_main__")
  File "<frozen runpy>", line 287, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "/builddir/build/BUILD/xpra-5.0.10-build/xpra-5.0.10/setup.py", line 2502, in <module>
    setup_options["ext_modules"] = cythonize(ext_modules,
                                   ~~~~~~~~~^^^^^^^^^^^^^
                                             nthreads=nthreads,
                                             ^^^^^^^^^^^^^^^^^^
                                             gdb_debug=debug_ENABLED,
                                             ^^^^^^^^^^^^^^^^^^^^^^^^
                                             compiler_directives=compiler_directives,
                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                             )
                                             ^
  File "/usr/lib64/python3.14/site-packages/Cython/Build/Dependencies.py", line 1134, in cythonize
    pool = multiprocessing.Pool(
        nthreads, initializer=_init_multiprocessing_helper)
  File "/usr/lib64/python3.14/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
                context=self.get_context())
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 215, in __init__
    self._repopulate_pool()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 306, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
                                        self._processes,
                                        ^^^^^^^^^^^^^^^^
    ...<3 lines>...
                                        self._maxtasksperchild,
                                        ^^^^^^^^^^^^^^^^^^^^^^^
                                        self._wrap_exception)
                                        ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/pool.py", line 329, in _repopulate_pool_static
    w.start()
    ~~~~~~~^^
  File "/usr/lib64/python3.14/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ~~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/context.py", line 300, in _Popen
    return Popen(process_obj)
  File "/usr/lib64/python3.14/multiprocessing/popen_forkserver.py", line 35, in __init__
    super().__init__(process_obj)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/popen_fork.py", line 20, in __init__
    self._launch(process_obj)
    ~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/usr/lib64/python3.14/multiprocessing/popen_forkserver.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/usr/lib64/python3.14/multiprocessing/spawn.py", line 164, in get_preparation_data
    _check_not_importing_main()
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/multiprocessing/spawn.py", line 140, in _check_not_importing_main
    raise RuntimeError('''
    ...<16 lines>...
    ''')
RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

        To fix this issue, refer to the "Safe importing of main module"
        section in https://docs.python.org/3/library/multiprocessing.html
        
'nvcc --version' failed with return code 127

From https://docs.python.org/dev/whatsnew/3.14.html#incompatible-changes:
On platforms other than macOS and Windows, the default start method for multiprocessing and ProcessPoolExecutor switches from fork to forkserver.

The testing takes place in Copr, where you can see the logs from the latest build:
https://copr.fedorainfracloud.org/coprs/g/python/python3.14/package/xpra/

Reproducible: Always