Make wxWebViewChromium work with wxGTK3 and X11

Make browser creation actually work by postponing it until the host
window is realized and so has a valid X11 Window.

Remove unnecessary code manually creating GTK widget and just use the
standard wxGTK wxWindow instead.

Add code for setting the visual compatible with CEF to avoid X11 errors,
see https://github.com/chromiumembedded/cef/issues/3564, with many
thanks to Jiří Janoušek for finding and solving this problem originally.

Adjust the sample to handle wxWebView::Create() failure (not very
gracefully, but still better than just crashing) and to avoid using it
until it is fully created.

Update documentation to mention GTK limitations.

include/wx/webview_chromium.h

@@ -118,10 +118,18 @@ public:
     //Virtual Filesystem Support
     virtual void RegisterHandler(wxSharedPtr<wxWebViewHandler> handler) override;
 
+#ifdef __WXGTK__
+    virtual void GTKHandleRealized() override;
+#endif
+
 protected:
     virtual void DoSetPage(const wxString& html, const wxString& baseUrl) override;
 
 private:
+    // Actually create the browser: this can only be done once the window is
+    // created in wxGTK.
+    bool DoCreateBrowser(const wxString& url);
+
     //History related variables, we currently use our own implementation
     wxVector<wxSharedPtr<wxWebViewHistoryItem> > m_historyList;
     int m_historyPosition;
@@ -142,6 +150,10 @@ private:
     friend class ClientHandler;
     ClientHandler* m_clientHandler;
 
+#ifdef __WXGTK__
+    wxString m_url;
+#endif
+
     friend class wxWebViewChromiumModule;
     static bool ms_cefInitialized;
 

interface/wx/webview_chromium.h

@@ -64,6 +64,23 @@
 
     Only Microsoft Visual C++ 2015 or newer can be used to build wxWebViewChromium.
 
+    __Linux with GTK__
+
+    wxWebviewChromium currently only supports X11 and not Wayland, i.e. you
+    need to either ensure that `GDK_BACKEND` environment variable is set to
+    "x11" before running your program using it or call
+
+    @code
+    gdk_set_allowed_backends("x11")
+    @endcode
+
+    in your application code.
+
+    Moreover, the actual browser is only created once the window is shown, and
+    can't be used until then. You can bind an event handler for wxEVT_CREATE to
+    know when it is usable.
+
+
     __Mac OS X Platform__
 
     OS X 10.9 or above is required.

samples/webview/webview.cpp

@@ -460,14 +460,17 @@ WebFrame::WebFrame(const wxString& url, bool isMain, wxWebViewWindowFeatures* wi
         m_browser->RegisterHandler(wxSharedPtr<wxWebViewHandler>(new AdvancedWebViewHandler()));
     }
 #endif
-    m_browser->Create(this, wxID_ANY, url, wxDefaultPosition,
+    if ( !m_browser->Create(this, wxID_ANY, url, wxDefaultPosition,
 #if defined(wxWEBVIEW_SAMPLE_CHROMIUM) && defined(__WXOSX__)
         // OSX implementation currently cannot handle the default size
         wxSize(800, 600)
 #else
         wxDefaultSize
 #endif
-    );
+    ) )
+    {
+        wxLogFatalError("Failed to create wxWebView");
+    }
 
     topsizer->Add(m_browser, wxSizerFlags().Expand().Proportion(1));
 
@@ -476,7 +479,14 @@ WebFrame::WebFrame(const wxString& url, bool isMain, wxWebViewWindowFeatures* wi
         // Log backend information
         wxLogMessage("Backend: %s Version: %s", m_browser->GetClassInfo()->GetClassName(),
             wxWebView::GetBackendVersionInfo().ToString());
-        wxLogMessage("User Agent: %s", m_browser->GetUserAgent());
+
+        // Chromium backend can't be used immediately after creation, so wait
+        // until the browser is created before calling GetUserAgent().
+        m_browser->Bind(wxEVT_CREATE, [this](wxWindowCreateEvent& event) {
+            wxLogMessage("User Agent: %s", m_browser->GetUserAgent());
+
+            event.Skip();
+        });
 
 #ifndef __WXMAC__
         //We register the wxfs:// protocol for testing purposes