Apply system background color for top level window

Repaint top level window with system background color when it shows up
first time. The system background color will be affected by dark or
light mode settings in windows

Fixes: QTBUG-106583
Pick-to: 6.4
Change-Id: I9205335540e74e90bb068e30fc3d4db037fd580f
Reviewed-by: Yuhang Zhao <2546789017@qq.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -31,6 +31,7 @@
 #include <private/qguiapplication_p.h>
 #include <private/qhighdpiscaling_p.h>
 #include <qpa/qwindowsysteminterface.h>
+#include <qpa/qplatformtheme.h>
 
 #include <QtCore/qdebug.h>
 #include <QtCore/qlibraryinfo.h>
@@ -2301,9 +2302,22 @@ static inline bool isSoftwareGl()
 }
 
 bool QWindowsWindow::handleWmPaint(HWND hwnd, UINT message,
-                                         WPARAM, LPARAM, LRESULT *result)
+                                   WPARAM wParam, LPARAM, LRESULT *result)
 {
     if (message == WM_ERASEBKGND) { // Backing store - ignored.
+        if (!m_firstBgDraw && QWindowsIntegration::instance()->darkModeHandling().testFlag(QWindowsApplication::DarkModeStyle)) {
+            // Get system background color
+            const QColor bgColor = QGuiApplicationPrivate::platformTheme()->palette()->color(QPalette::Window);
+            HBRUSH bgBrush = CreateSolidBrush(RGB(bgColor.red(), bgColor.green(), bgColor.blue()));
+            // Fill rectangle with system background color
+            RECT rc;
+            auto hdc = reinterpret_cast<HDC>(wParam);
+            GetClientRect(hwnd, &rc);
+            FillRect(hdc, &rc, bgBrush);
+            DeleteObject(bgBrush);
+            // Brush the window with system background color only for first time
+            m_firstBgDraw = true;
+        }
         *result = 1;
         return true;
     }

src/plugins/platforms/windows/qwindowswindow.h

@@ -377,6 +377,7 @@ private:
     HICON m_iconBig = nullptr;
     void *m_surface = nullptr;
     int m_savedDpi = 96;
+    bool m_firstBgDraw = false;
 
     static bool m_screenForGLInitialized;