Windows QPA: Cache "forcedScreenForGLWindow" to avoid overhead

The fix for QTBUG-50371 caused an overhead when you hover over a
secondary window like a tool tip, menu or combo box due to the
forcedScreenForGLWindow() function being called, which loads
dx9.dll and sub-dlls and unloads them afterwards. This fix
caches the required info on the first call, and only refreshes
it when required by a display/settings change.

Fixes: QTBUG-73008
Change-Id: Ie604ba4034ad8041b971f5aa46bd43ae03decd55
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1115,8 +1115,10 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
     case QtWindows::DisplayChangedEvent:
         if (QWindowsTheme *t = QWindowsTheme::instance())
             t->displayChanged();
+        QWindowsWindow::displayChanged();
         return d->m_screenManager.handleDisplayChange(wParam, lParam);
     case QtWindows::SettingChangedEvent:
+        QWindowsWindow::settingsChanged();
         return d->m_screenManager.handleScreenChanges();
     default:
         break;

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -551,12 +551,6 @@ static QScreen *screenForName(const QWindow *w, const QString &name)
     return winScreen;
 }
 
-static QScreen *forcedScreenForGLWindow(const QWindow *w)
-{
-    const QString forceToScreen = GpuDescription::detect().gpuSuitableScreen;
-    return forceToScreen.isEmpty() ? nullptr : screenForName(w, forceToScreen);
-}
-
 static QPoint calcPosition(const QWindow *w, const QWindowCreationContextPtr &context, const QMargins &invMargins)
 {
     const QPoint orgPos(context->frameX - invMargins.left(), context->frameY - invMargins.top());
@@ -565,7 +559,7 @@ static QPoint calcPosition(const QWindow *w, const QWindowCreationContextPtr &co
         return orgPos;
 
     // Workaround for QTBUG-50371
-    const QScreen *screenForGL = forcedScreenForGLWindow(w);
+    const QScreen *screenForGL = QWindowsWindow::forcedScreenForGLWindow(w);
     if (!screenForGL)
         return orgPos;
 
@@ -1359,6 +1353,28 @@ void QWindowsWindow::setDropSiteEnabled(bool dropEnabled)
 #endif // QT_CONFIG(clipboard) && QT_CONFIG(draganddrop)
 }
 
+bool QWindowsWindow::m_screenForGLInitialized = false;
+
+void QWindowsWindow::displayChanged()
+{
+    m_screenForGLInitialized = false;
+}
+
+void QWindowsWindow::settingsChanged()
+{
+    m_screenForGLInitialized = false;
+}
+
+QScreen *QWindowsWindow::forcedScreenForGLWindow(const QWindow *w)
+{
+    static QString forceToScreen;
+    if (!m_screenForGLInitialized) {
+        forceToScreen = GpuDescription::detect().gpuSuitableScreen;
+        m_screenForGLInitialized = true;
+    }
+    return forceToScreen.isEmpty() ? nullptr : screenForName(w, forceToScreen);
+}
+
 // Returns topmost QWindowsWindow ancestor even if there are embedded windows in the chain.
 // Returns this window if it is the topmost ancestor.
 QWindow *QWindowsWindow::topLevelOf(QWindow *w)

src/plugins/platforms/windows/qwindowswindow.h

@@ -297,6 +297,9 @@ public:
     void handleHidden();
     void handleCompositionSettingsChanged();
 
+    static void displayChanged();
+    static void settingsChanged();
+    static QScreen *forcedScreenForGLWindow(const QWindow *w);
     static QWindowsWindow *windowsWindowOf(const QWindow *w);
     static QWindow *topLevelOf(QWindow *w);
     static inline void *userDataOf(HWND hwnd);
@@ -377,6 +380,8 @@ private:
     HICON m_iconBig = 0;
     void *m_surface = nullptr;
 
+    static bool m_screenForGLInitialized;
+
 #if QT_CONFIG(vulkan)
     // note: intentionally not using void * in order to avoid breaking x86
     VkSurfaceKHR m_vkSurface = 0;