Improve waiting for browser close to avoid crash on shutdown

Ensure that CefLifeSpanHandler::OnBeforeClose() is called before
returning from wxWebViewChromium dtor, as otherwise we may try to shut
down CEF before the browser object is destroyed.

Under MSW it's enough to destroy the window ourselves to make this
happen, but under GTK/X11 we need to manually pump the messages, as
OnBeforeClose() is only called when the X event notifying about the
actual window destruction arrives.

Also make the code cleaner by using flags in wxWebViewChromiumImplData
instead of reusing m_clientHandler pointer to indicate that DoClose()
was called.

src/common/webview_chromium.cpp

@@ -10,6 +10,7 @@
 
 #include "wx/webview.h"
 #include "wx/webview_chromium.h"
+#include "wx/evtloop.h"
 #include "wx/filename.h"
 #include "wx/filesys.h"
 #include "wx/rtti.h"
@@ -79,6 +80,10 @@ struct wxWebViewChromiumImplData
     // URL passed to Create() as we can't use it there directly.
     wxString m_initialURL;
 #endif // __WXGTK__
+
+    // These flags are used when destroying wxWebViewChromium, see its dtor.
+    bool m_calledDoClose = false;
+    bool m_calledOnBeforeClose = false;
 };
 
 // ----------------------------------------------------------------------------
@@ -484,18 +489,53 @@ wxWebViewChromium::~wxWebViewChromium()
     {
         wxLogTrace(TRACE_CEF, "closing browser");
 
-        // Preserve the original pointer.
-        const auto clientHandler = m_clientHandler;
+        const auto handle = m_clientHandler->GetWindowHandle();
 
         constexpr bool forceClose = true;
         m_clientHandler->GetBrowser()->GetHost()->CloseBrowser(forceClose);
+        m_clientHandler->Release();
 
-        // This should be set to nullptr from ClientHandler::DoClose().
-        while ( m_clientHandler )
+        // We need to wait until the browser is really closed, which happens
+        // asynchronously, as otherwise we could exit the program and call
+        // CefShutdown() before ClientHandler is destroyed, which would kill
+        // the program with "Object reference incorrectly held at CefShutdown"
+        // error message.
+
+        // First wait until our ClientHandler::DoClose() is called: it will
+        // reset our m_clientHandler when this happens.
+        while ( !m_implData->m_calledDoClose )
             CefDoMessageLoopWork();
 
-        clientHandler->Release();
+#ifdef __WXMSW__
+        // Under MSW we need to destroy the window: if we don't do this,
+        // OnBeforeClose() won't get called at all, no matter how many messages
+        // we dispatch or how many times we call CefDoMessageLoopWork().
+        ::DestroyWindow(handle);
+
+        while ( !m_implData->m_calledOnBeforeClose )
+            CefDoMessageLoopWork();
+#elif defined(__WXGTK__)
+        // This doesn't seem to be necessary, the window gets destroyed on its
+        // own when dispatching the events, but still do it as it might speed
+        // up the shutdown and we can also do this if there is no active event
+        // loop (which should never happen, of course).
+        ::XDestroyWindow(wxGetX11Display(), handle);
+
+        if ( const auto eventLoop = wxEventLoop::GetActive() )
+        {
+            while ( !m_implData->m_calledOnBeforeClose )
+            {
+                // Under GTK just calling CefDoMessageLoopWork() isn't enough,
+                // we need to dispatch the lower level (e.g. X11) events
+                // notifying CEF about the window destruction.
+                eventLoop->DispatchTimeout(10);
+            }
+        }
+        //else: we're going to crash on shutdown, but what else can we do?
+#endif
     }
+
+    delete m_implData;
 }
 
 bool wxWebViewChromium::InitCEF()
@@ -964,7 +1004,7 @@ bool ClientHandler::DoClose(CefRefPtr<CefBrowser> WXUNUSED(browser))
 {
     TRACE_CEF_FUNCTION();
 
-    m_webview.m_clientHandler = nullptr;
+    m_webview.m_implData->m_calledDoClose = true;
 
     return false;
 }
@@ -973,6 +1013,8 @@ void ClientHandler::OnBeforeClose(CefRefPtr<CefBrowser> browser)
 {
     TRACE_CEF_FUNCTION();
 
+    m_webview.m_implData->m_calledOnBeforeClose = true;
+
     if ( browser->GetIdentifier() == m_browserId )
     {
         m_browser = nullptr;