Create the browser immediately after Initialize() instead of via OnContextInitialized

Revert the browser-creation-timing workaround. Under cefpython's single-threaded
message loop the Chrome runtime initializes the browser context during
CefInitialize(), so CreateBrowserSync() can be called immediately after
Initialize() returns and runs synchronously. This is the pre-existing pattern and
keeps the examples and unit-test harness simple, with no global client callback,
no deferred-embed helpers, and no polling.

- cefpython.pyx: drop the CreateBrowserSync() guard that required the context
  callback and drop OnContextInitialized from the SetGlobalClientCallback allow-list.
- Remove the now-unused OnContextInitialized wiring entirely: the
  g_context_initialized flag (cefpython.pyx), the
  BrowserProcessHandler_OnContextInitialized function (browser_process_handler.pyx),
  and its call in subprocess/cefpython_app.cpp.
- Restore immediate browser creation in all examples and in the main/osr/issue517
  unit tests.

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

Author: linesight

examples/gtk2.py

@@ -33,28 +33,11 @@
 MESSAGE_LOOP_CEF = 2  # Pass --message-loop-cef flag to script on Linux
 g_message_loop = None
 
-# Under CEF's Chrome runtime the browser context initializes asynchronously,
-# so a browser can only be created after OnContextInitialized has fired. These
-# module-level helpers coordinate that: embed_browser() defers itself until the
-# context is ready, and the callback below runs any deferred embed.
-g_context_initialized = False
-g_pending_embed = None
-
-
-def _on_context_initialized():
-    global g_context_initialized
-    g_context_initialized = True
-    if g_pending_embed is not None:
-        g_pending_embed()
-
 
 def main():
     check_versions()
     sys.excepthook = cef.ExceptHook  # To shutdown all CEF processes on error
     configure_message_loop()
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized",
-                                _on_context_initialized)
     cef.Initialize()
     gobject.threads_init()
     Gtk2Example()
@@ -125,13 +108,6 @@ def __init__(self):
             gobject.timeout_add(10, self.on_timer)
 
     def embed_browser(self):
-        global g_pending_embed
-        if not g_context_initialized:
-            # Chrome runtime initializes the browser context asynchronously;
-            # embed once OnContextInitialized fires (see main()).
-            g_pending_embed = self.embed_browser
-            return
-        g_pending_embed = None
         windowInfo = cef.WindowInfo()
         size = self.main_window.get_size()
         rect = [0, 0, size[0], size[1]]

examples/gtk3.py

@@ -34,21 +34,6 @@
     from gi.repository import GdkX11
 
 
-# Under CEF's Chrome runtime the browser context initializes asynchronously,
-# so a browser can only be created after OnContextInitialized has fired. These
-# module-level helpers coordinate that: embed_browser() defers itself until the
-# context is ready, and the callback below runs any deferred embed.
-g_context_initialized = False
-g_pending_embed = None
-
-
-def _on_context_initialized():
-    global g_context_initialized
-    g_context_initialized = True
-    if g_pending_embed is not None:
-        g_pending_embed()
-
-
 def main():
     print("[gkt3.py] CEF Python {ver}".format(ver=cef.__version__))
     print("[gkt3.py] Python {ver} {arch}".format(
@@ -62,9 +47,6 @@ def main():
         # > Python[57738:d07] _createMenuRef called with existing principal
         # > MenuRef already associated with menu
         sys.excepthook = cef.ExceptHook  # To shutdown CEF processes on error
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized",
-                                _on_context_initialized)
     cef.Initialize()
     app = Gtk3Example()
     SystemExit(app.run(sys.argv))
@@ -130,13 +112,6 @@ def on_activate(self, *_):
         self.window.resize(*self.window.get_default_size())
 
     def embed_browser(self):
-        global g_pending_embed
-        if not g_context_initialized:
-            # Chrome runtime initializes the browser context asynchronously;
-            # embed once OnContextInitialized fires (see main()).
-            g_pending_embed = self.embed_browser
-            return
-        g_pending_embed = None
         window_info = cef.WindowInfo()
         # TODO: on Mac pass rect[x, y, width, height] to SetAsChild
         window_info.SetAsChild(self.get_handle())

examples/hello_world.py

@@ -18,20 +18,9 @@ def main():
     check_versions()
     sys.excepthook = cef.ExceptHook  # shut down all CEF processes on error
     settings = {}
-
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so the browser must be created from the
-        # OnContextInitialized callback rather than immediately after
-        # cef.Initialize(). This matches CEF's cefsimple sample.
-        cef.CreateBrowserSync(url="https://www.google.com/",
-                              window_title="Hello World!")
-
-    # Register the callback before cef.Initialize(): OnContextInitialized can
-    # fire during cef.Initialize() itself.
-    cef.SetGlobalClientCallback("OnContextInitialized",
-                                on_context_initialized)
     cef.Initialize(settings=settings)
+    cef.CreateBrowserSync(url="https://www.google.com/",
+                          window_title="Hello World!")
     cef.MessageLoop()
     cef.Shutdown()
 

examples/pysdl2.py

@@ -104,18 +104,6 @@ def die(msg):
             "       To install type: pip install pyobjc")
 
 
-# Under CEF's Chrome runtime the browser context initializes asynchronously,
-# so the (off-screen) browser can only be created after OnContextInitialized
-# has fired. This flag is set by the callback registered before
-# cef.Initialize().
-g_context_initialized = False
-
-
-def _on_context_initialized():
-    global g_context_initialized
-    g_context_initialized = True
-
-
 def main():
     """
     Parses input, initializes everything and then runs the main loop of the
@@ -205,9 +193,6 @@ def main():
         # Tweaking OSR performance (Issue #240)
         "windowless_frame_rate": frameRate
     }
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized",
-                                _on_context_initialized)
     cef.Initialize(settings={"windowless_rendering_enabled": True},
                    switches=switches)
 
@@ -311,14 +296,6 @@ def _detect_device_scale_factor(window, renderer):
     # RenderHandler instances, creating SDL windows for popups lazily.
     renderHandler = MetaRenderHandler(renderer, width, height - headerHeight,
                                       deviceScaleFactor, rendererFlags)
-    # Create the browser instance. Unlike the windowed examples (which embed
-    # the browser from an event callback), this linear off-screen example
-    # creates it inline, so ensure the CEF context has finished initializing
-    # first. On most platforms it already has by this point (it initializes
-    # during cef.Initialize()); otherwise let the message loop run until
-    # OnContextInitialized fires.
-    while not g_context_initialized:
-        cef.MessageLoopWork()
     # Create the browser instance
     browser = cef.CreateBrowserSync(window_info,
                                     url="https://www.google.com/",

examples/qt.py

@@ -74,21 +74,6 @@
 HEIGHT = 600
 
 
-# Under CEF's Chrome runtime the browser context initializes asynchronously,
-# so a browser can only be created after OnContextInitialized has fired. These
-# module-level helpers coordinate that: embedBrowser() defers itself until the
-# context is ready, and the callback below runs any deferred embed.
-g_context_initialized = False
-g_pending_embed = None
-
-
-def _on_context_initialized():
-    global g_context_initialized
-    g_context_initialized = True
-    if g_pending_embed is not None:
-        g_pending_embed()
-
-
 def main():
     check_versions()
     sys.excepthook = cef.ExceptHook  # To shutdown all CEF processes on error
@@ -107,9 +92,6 @@ def main():
         "devtools": True,
     }
 
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized",
-                                _on_context_initialized)
     cef.Initialize(settings)
     app = CefApplication(sys.argv)
     main_window = MainWindow()
@@ -266,13 +248,6 @@ def focusOutEvent(self, event):
             self.browser.SetFocus(False)
 
     def embedBrowser(self):
-        global g_pending_embed
-        if not g_context_initialized:
-            # Chrome runtime initializes the browser context asynchronously;
-            # embed once OnContextInitialized fires (see main()).
-            g_pending_embed = self.embedBrowser
-            return
-        g_pending_embed = None
         if LINUX and PYQT5:
             # On Linux with PyQt5, QX11EmbedContainer is gone; the Qt-native
             # equivalent is to host CEF in a QWindow (hidden_window) wrapped in

examples/screenshot.py

@@ -97,15 +97,8 @@ def main():
         # Tweaking OSR performance (Issue #240)
         "windowless_frame_rate": 30,  # Default frame rate in CEF is 30
     }
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so create the browser here rather than right after
-        # cef.Initialize().
-        create_browser(browser_settings)
-
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized", on_context_initialized)
     cef.Initialize(settings=settings, switches=switches)
+    create_browser(browser_settings)
     cef.MessageLoop()
     cef.Shutdown()
     print("[screenshot.py] Opening screenshot with default application")

examples/snippets/cookies.py

@@ -7,19 +7,13 @@
 
 
 def main():
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so create the browser here rather than right after
-        # cef.Initialize().
-        browser = cef.CreateBrowserSync(
-            url="https://www.google.com/",
-            window_title="Cookies")
-        browser.SetClientHandler(LoadHandler())
-
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized", on_context_initialized)
     cef.Initialize()
+    browser = cef.CreateBrowserSync(
+        url="https://www.google.com/",
+        window_title="Cookies")
+    browser.SetClientHandler(LoadHandler())
     cef.MessageLoop()
+    del browser
     cef.Shutdown()
 
 

examples/snippets/crossdomain_bindings.py

@@ -37,24 +37,17 @@
 
 def main():
     print(__doc__)
-
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so create the browser here rather than right after
-        # cef.Initialize().
-        browser = cef.CreateBrowserSync(
-            url=TARGET_URL,
-            window_title="Cross-domain JS binding test")
-        handler = CrossDomainHandler(browser)
-        browser.SetClientHandler(handler)
-        bindings = cef.JavascriptBindings()
-        bindings.SetFunction("OnTargetPageReady", handler["_OnTargetPageReady"])
-        browser.SetJavascriptBindings(bindings)
-
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized", on_context_initialized)
     cef.Initialize()
+    browser = cef.CreateBrowserSync(url=TARGET_URL,
+                                    window_title="Cross-domain JS binding test")
+    handler = CrossDomainHandler(browser)
+    browser.SetClientHandler(handler)
+    bindings = cef.JavascriptBindings()
+    bindings.SetFunction("OnTargetPageReady", handler["_OnTargetPageReady"])
+    browser.SetJavascriptBindings(bindings)
     cef.MessageLoop()
+    del handler
+    del browser
     cef.Shutdown()
 
 

examples/snippets/javascript_bindings.py

@@ -38,22 +38,16 @@
 
 
 def main():
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so create the browser here rather than right after
-        # cef.Initialize().
-        browser = cef.CreateBrowserSync(url=cef.GetDataUrl(g_htmlcode),
-                                        window_title="Javascript Bindings")
-        browser.SetClientHandler(LoadHandler())
-        bindings = cef.JavascriptBindings()
-        bindings.SetFunction("py_function", py_function)
-        bindings.SetFunction("py_callback", py_callback)
-        browser.SetJavascriptBindings(bindings)
-
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized", on_context_initialized)
     cef.Initialize()
+    browser = cef.CreateBrowserSync(url=cef.GetDataUrl(g_htmlcode),
+                                    window_title="Javascript Bindings")
+    browser.SetClientHandler(LoadHandler())
+    bindings = cef.JavascriptBindings()
+    bindings.SetFunction("py_function", py_function)
+    bindings.SetFunction("py_callback", py_callback)
+    browser.SetJavascriptBindings(bindings)
     cef.MessageLoop()
+    del browser
     cef.Shutdown()
 
 

examples/snippets/javascript_errors.py

@@ -40,17 +40,10 @@
 
 
 def main():
-    def on_context_initialized():
-        # Under CEF's Chrome runtime the browser context initializes
-        # asynchronously, so create the browser here rather than right after
-        # cef.Initialize().
-        browser = cef.CreateBrowserSync(url=cef.GetDataUrl(g_htmlcode),
-                                        window_title="Javascript Errors")
-        browser.SetClientHandler(DisplayHandler())
-
-    # Register before cef.Initialize(); OnContextInitialized may fire during it.
-    cef.SetGlobalClientCallback("OnContextInitialized", on_context_initialized)
     cef.Initialize()
+    browser = cef.CreateBrowserSync(url=cef.GetDataUrl(g_htmlcode),
+                                    window_title="Javascript Errors")
+    browser.SetClientHandler(DisplayHandler())
     cef.MessageLoop()
     cef.Shutdown()