From 3ce2e00891eae92d98241ff4c9cb0c2fbf8ea10a Mon Sep 17 00:00:00 2001
From: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>
Date: Tue, 25 Oct 2022 13:47:26 +0200
Subject: [PATCH] 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
Change-Id: I9205335540e74e90bb068e30fc3d4db037fd580f
Reviewed-by: Yuhang Zhao <2546789017@qq.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 2991c66b75612dfb11dbba166dd08b2376b42102)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/plugins/platforms/windows/qwindowswindow.cpp | 16 +++++++++++++++-
 src/plugins/platforms/windows/qwindowswindow.h   |  1 +
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index a795328bdf9..dc7ecc25700 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/src/plugins/platforms/windows/qwindowswindow.cpp
@@ -33,6 +33,7 @@
 #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>
@@ -2303,9 +2304,22 @@ 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;
     }
diff --git a/src/plugins/platforms/windows/qwindowswindow.h b/src/plugins/platforms/windows/qwindowswindow.h
index 29dfbdb856b..eee13c2281c 100644
--- a/src/plugins/platforms/windows/qwindowswindow.h
+++ b/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;