Offscreen QPA: Use fusion style always

Prevent it from crashing when naively using it for example
on Windows, which defaults to the Windows Vista style operating
on native window handles.

Change-Id: I7b1dfb00a6b6860d0f0a134653ce1142b45959ec
Reviewed-by: Sami Nurmenniemi <sami.nurmenniemi@qt.io>
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/offscreen/qoffscreenintegration.cpp

@@ -61,6 +61,7 @@
 #include <QtGui/private/qguiapplication_p.h>
 #include <qpa/qplatforminputcontextfactory_p.h>
 #include <qpa/qplatforminputcontext.h>
+#include <qpa/qplatformtheme.h>
 
 #include <qpa/qplatformservices.h>
 
@@ -167,6 +168,37 @@ QAbstractEventDispatcher *QOffscreenIntegration::createEventDispatcher() const
 #endif
 }
 
+static QString themeName() { return QStringLiteral("offscreen"); }
+
+QStringList QOffscreenIntegration::themeNames() const
+{
+    return QStringList(themeName());
+}
+
+// Restrict the styles to "fusion" to prevent native styles requiring native
+// window handles (eg Windows Vista style) from being used.
+class OffscreenTheme : public QPlatformTheme
+{
+public:
+    OffscreenTheme() {}
+
+    QVariant themeHint(ThemeHint h) const override
+    {
+        switch (h) {
+        case StyleNames:
+            return QVariant(QStringList(QStringLiteral("fusion")));
+        default:
+            break;
+        }
+        return QPlatformTheme::themeHint(h);
+    }
+};
+
+QPlatformTheme *QOffscreenIntegration::createPlatformTheme(const QString &name) const
+{
+    return name == themeName() ? new OffscreenTheme() : nullptr;
+}
+
 QPlatformFontDatabase *QOffscreenIntegration::fontDatabase() const
 {
     return m_fontDatabase.data();

src/plugins/platforms/offscreen/qoffscreenintegration.h

@@ -69,6 +69,9 @@ public:
     QPlatformFontDatabase *fontDatabase() const Q_DECL_OVERRIDE;
     QAbstractEventDispatcher *createEventDispatcher() const Q_DECL_OVERRIDE;
 
+    QStringList themeNames() const;
+    QPlatformTheme *createPlatformTheme(const QString &name) const;
+
     static QOffscreenIntegration *createOffscreenIntegration();
 
 private: