Never call QtWebEngine::initialize from DllMain

On windows, calling QtWebEngine::initialize from DllMain may crash,
depending on what QPA backend is used.

We make sure to only add a qAddPreRoutine initializing QtWebEngine
if there is no instance of QCoreApplication.
By doing so, we support linking to QtWebEngineWidget from a plugin,
while still initializing the WebEngine automatically when linked to
the main application binary.

Task-number: QTBUG-46720
Reviewed-by: Jocelyn Turcotte (Woboq GmbH) <jturcotte@woboq.com>
(cherry picked from commit 6a8d99ab00ace2084820547572dc05ac8ad2bc5a)
Change-Id: Id3b2c1e21e01808cef124f8ece6b247b1ba613cb
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/core/api/qtwebenginecoreglobal.cpp b/src/core/api/qtwebenginecoreglobal.cpp
index 0e857d7d9..e60e6139e 100644
--- a/src/core/api/qtwebenginecoreglobal.cpp
+++ b/src/core/api/qtwebenginecoreglobal.cpp
@@ -75,6 +75,11 @@ QWEBENGINE_PRIVATE_EXPORT void initialize()
         qFatal("QtWebEngine::initialize() must be called after the construction of the application object.");
         return;
     }
+
+    // Bail out silently if the user did not construct a QGuiApplication.
+    if (!qobject_cast<QGuiApplication *>(app))
+        return;
+
     if (app->thread() != QThread::currentThread()) {
         qFatal("QtWebEngine::initialize() must be called from the Qt gui thread.");
         return;