build(cython): drop platform_cimports.pxi workaround; keep IF/DEF usage consistent

Revert the fork-only partial IF/DEF conversions the CEF 147 modernization
introduced, per reviewer request (r3512025176): keep the codebase uniform on
IF; a complete IF/DEF migration is out of scope for this PR.

- Delete the fork-only src/platform_cimports.pxi and
  src/extern/cef/platform_cimports.pxi (both absent from upstream cefpython147).
  Restore upstream's inline per-file `IF UNAME_SYSNAME` platform cimports in
  cefpython.pyx and cef_app/cef_browser/cef_browser_static/cef_platform.pxd,
  plus the CEF 147 `cimport sandbox_linux`.
- utils.pyx: revert GetSystemError() to its original `IF UNAME_SYSNAME` form,
  dropping the inline `cdef extern from *` C block.
- browser.pyx: restore `IF UNAME_SYSNAME == "Linux": cimport x11`.
- window_utils_win.pyx: restore the IsWindowHandle IF/ELSE guard.

Verified on Windows: clean build + 85/85 unit tests. Cython front-end also
transpiles clean for Linux and Darwin (forced via -E UNAME_SYSNAME); C++
compile/link on those platforms is left to CI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: linesight

src/browser.pyx

@@ -8,6 +8,8 @@ cimport cef_types
 from libc.stdint cimport uint32_t, int64_t
 from libcpp cimport nullptr
 from cef_types cimport cef_state_t
+IF UNAME_SYSNAME == "Linux":
+    cimport x11
 
 # cef_mouse_button_type_t, SendMouseClickEvent().
 MOUSEBUTTON_LEFT = cef_types.MBT_LEFT

src/cefpython.pyx

@@ -212,11 +212,21 @@ ctypedef uintptr_t WindowHandle
 # noinspection PyUnresolvedReferences
 cimport ctime
 
-include "platform_cimports.pxi"
+IF UNAME_SYSNAME == "Windows":
+    from windows cimport *
+    from dpi_aware_win cimport *
+ELIF UNAME_SYSNAME == "Linux":
+    from linux cimport *
+ELIF UNAME_SYSNAME == "Darwin":
+    from mac cimport *
 
 from cpp_utils cimport *
 from task cimport *
 
+IF UNAME_SYSNAME == "Linux":
+    cimport x11
+    cimport sandbox_linux
+
 from cef_string cimport *
 cdef extern from *:
     # noinspection PyUnresolvedReferences

src/extern/cef/cef_app.pxd

@@ -5,12 +5,19 @@
 # Circular imports are allowed in form "cimport ...",
 # but won't work if you do "from ... cimport *".
 
-include "platform_cimports.pxi"
+include "compile_time_constants.pxi"
 
 from cef_types cimport CefSettings
 from cef_ptr cimport CefRefPtr
 from libcpp cimport bool as cpp_bool
 
+IF UNAME_SYSNAME == "Windows":
+    from cef_win cimport CefMainArgs
+ELIF UNAME_SYSNAME == "Linux":
+    from cef_linux cimport CefMainArgs
+ELIF UNAME_SYSNAME == "Darwin":
+    from cef_mac cimport CefMainArgs
+
 cdef extern from "include/cef_app.h":
 
     cdef cppclass CefApp:

src/extern/cef/cef_browser.pxd

@@ -2,7 +2,14 @@
 # All rights reserved. Licensed under BSD 3-clause license.
 # Project website: https://github.com/cztomczak/cefpython
 
-include "platform_cimports.pxi"
+include "compile_time_constants.pxi"
+
+IF UNAME_SYSNAME == "Windows":
+    from cef_win cimport CefWindowHandle, CefWindowInfo
+ELIF UNAME_SYSNAME == "Linux":
+    from cef_linux cimport CefWindowHandle, CefWindowInfo
+ELIF UNAME_SYSNAME == "Darwin":
+    from cef_mac cimport CefWindowHandle, CefWindowInfo
 
 from cef_ptr cimport CefRefPtr
 from cef_string cimport CefString

src/extern/cef/cef_browser_static.pxd

@@ -2,7 +2,14 @@
 # All rights reserved. Licensed under BSD 3-clause license.
 # Project website: https://github.com/cztomczak/cefpython
 
-include "platform_cimports.pxi"
+include "compile_time_constants.pxi"
+
+IF UNAME_SYSNAME == "Windows":
+    from cef_win cimport CefWindowInfo
+ELIF UNAME_SYSNAME == "Linux":
+    from cef_linux cimport CefWindowInfo
+ELIF UNAME_SYSNAME == "Darwin":
+    from cef_mac cimport CefWindowInfo
 
 from libcpp cimport bool as cpp_bool
 from cef_ptr cimport CefRefPtr

src/extern/cef/cef_platform.pxd

@@ -2,4 +2,11 @@
 # All rights reserved. Licensed under BSD 3-clause license.
 # Project website: https://github.com/cztomczak/cefpython
 
-include "platform_cimports.pxi"
+include "compile_time_constants.pxi"
+
+IF UNAME_SYSNAME == "Windows":
+    from cef_win cimport *
+ELIF UNAME_SYSNAME == "Darwin":
+    from cef_mac cimport *
+ELIF UNAME_SYSNAME == "Linux":
+    from cef_linux cimport *

src/extern/cef/platform_cimports.pxi

@@ -63,24 +63,12 @@ cdef void NonCriticalError(object msg) except *:
     msg = "[Browser process] " + msg
     cef_log_error(PyStringToChar(msg))
 
-cdef extern from *:
-    """
-    #ifdef _WIN32
-    #include <windows.h>
-    static int _cefpy_get_win32_last_error(void) { return (int)GetLastError(); }
-    static int _cefpy_is_windows(void) { return 1; }
-    #else
-    static int _cefpy_get_win32_last_error(void) { return 0; }
-    static int _cefpy_is_windows(void) { return 0; }
-    #endif
-    """
-    cdef int _cefpy_get_win32_last_error() noexcept nogil
-    cdef int _cefpy_is_windows() noexcept nogil
-
 cpdef str GetSystemError():
-    if _cefpy_is_windows():
-        return "Error Code = %d" % _cefpy_get_win32_last_error()
-    return ""
+    IF UNAME_SYSNAME == "Windows":
+        cdef DWORD errorCode = GetLastError()
+        return "Error Code = %d" % errorCode
+    ELSE:
+        return ""
 
 cpdef py_bool IsFunctionOrMethod(object valueType):
     if (valueType == types.FunctionType

src/platform_cimports.pxi

@@ -143,7 +143,10 @@ class WindowUtils(object):
 
     @classmethod
     def IsWindowHandle(cls, WindowHandle windowHandle):
-        return bool(IsWindow(<HWND>windowHandle))
+        IF UNAME_SYSNAME == "Windows":
+            return bool(IsWindow(<HWND>windowHandle))
+        ELSE:
+            return False
 
     @classmethod
     def InstallX11ErrorHandlers(cls):