Windows QPA: Add setting of dark window borders

Implement dark mode support level 1: change the
window borders to dark and back, tracking the
activation of dark mode.

Task-number: QTBUG-72028
Change-Id: I6e8b31e7ee574f4d90438405f361cd940faee7fd
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1221,6 +1221,11 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
             auto nativeInterface =
                 static_cast<QWindowsNativeInterface *>(QWindowsIntegration::instance()->nativeInterface());
             emit nativeInterface->darkModeChanged(darkMode);
+            const auto options = QWindowsIntegration::instance()->options();
+            if ((options & QWindowsIntegration::DarkModeWindowFrames) != 0) {
+                for (QWindowsWindow *w : d->m_windows)
+                    w->setDarkBorder(QWindowsContextPrivate::m_darkMode);
+            }
         }
         return d->m_screenManager.handleScreenChanges();
     }

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -749,6 +749,11 @@ void WindowCreationData::fromWindow(const QWindow *w, const Qt::WindowFlags flag
     }
 }
 
+static inline bool shouldApplyDarkFrame(const QWindow *w)
+{
+    return w->isTopLevel() && !w->flags().testFlag(Qt::FramelessWindowHint);
+}
+
 QWindowsWindowData
     WindowCreationData::create(const QWindow *w, const WindowData &data, QString title) const
 {
@@ -816,6 +821,12 @@ QWindowsWindowData
         return result;
     }
 
+    if (QWindowsContext::isDarkMode()
+        && (QWindowsIntegration::instance()->options() & QWindowsIntegration::DarkModeWindowFrames) != 0
+        && shouldApplyDarkFrame(w)) {
+        QWindowsWindow::setDarkBorderToWindow(result.hwnd, true);
+    }
+
     if (mirrorParentWidth != 0) {
         context->obtainedPos.setX(mirrorParentWidth - context->obtainedSize.width()
                                   -  context->obtainedPos.x());
@@ -2892,6 +2903,39 @@ bool QWindowsWindow::isTopLevel() const
     return window()->isTopLevel() && !m_data.embedded;
 }
 
+enum : WORD {
+    DwmwaUseImmersiveDarkMode = 20,
+    DwmwaUseImmersiveDarkModeBefore20h1 = 19
+};
+
+static bool queryDarkBorder(HWND hwnd)
+{
+    BOOL result = FALSE;
+    const bool ok =
+        SUCCEEDED(DwmGetWindowAttribute(hwnd, DwmwaUseImmersiveDarkMode, &result, sizeof(result)))
+        || SUCCEEDED(DwmGetWindowAttribute(hwnd, DwmwaUseImmersiveDarkModeBefore20h1, &result, sizeof(result)));
+    if (!ok)
+        qWarning("%s: Unable to retrieve dark window border setting.", __FUNCTION__);
+    return result == TRUE;
+}
+
+bool QWindowsWindow::setDarkBorderToWindow(HWND hwnd, bool d)
+{
+    const BOOL darkBorder = d ? TRUE : FALSE;
+    const bool ok =
+        SUCCEEDED(DwmSetWindowAttribute(hwnd, DwmwaUseImmersiveDarkMode, &darkBorder, sizeof(darkBorder)))
+        || SUCCEEDED(DwmSetWindowAttribute(hwnd, DwmwaUseImmersiveDarkModeBefore20h1, &darkBorder, sizeof(darkBorder)));
+    if (!ok)
+        qWarning("%s: Unable to set dark window border.", __FUNCTION__);
+    return ok;
+}
+
+void QWindowsWindow::setDarkBorder(bool d)
+{
+    if (shouldApplyDarkFrame(window()) && queryDarkBorder(m_data.hwnd) != d)
+        setDarkBorderToWindow(m_data.hwnd, d);
+}
+
 QWindowsMenuBar *QWindowsWindow::menuBar() const
 {
     return m_menuBar.data();

src/plugins/platforms/windows/qwindowswindow.h

@@ -287,6 +287,9 @@ public:
     HWND handle() const override { return m_data.hwnd; }
     bool isTopLevel() const override;
 
+    static bool setDarkBorderToWindow(HWND hwnd, bool d);
+    void setDarkBorder(bool d);
+
     QWindowsMenuBar *menuBar() const;
     void setMenuBar(QWindowsMenuBar *mb);