Windows QPA: Update screen on child window DPI change

Windows does not send WM_DPICHANGED to child windows, which means
that the normal DPI change handling code does not run for QWindows
which are embedded in a foreign, non-Qt, window.

Add code which handles WM_DPICHANGED_AFTERPARENT. This event is
sent to all child windows, but not the top-level window. Call
checkForScreenChanged() here, similar to what the WM_DPICHANGED code
does.

This commit does not add code to resize the child window, since
it is uncertain if this is the responsibility of the window which
receives WM_DPICHANGED, or of each child window.

Done-with: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Task-number: QTBUG-103383
Change-Id: Icf85dd0afa806609dbbe0ffc36efbc5127962c39
Reviewed-by: <stefan.wastl@native-instruments.de>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 077eddb3e1aaba1517d67e7c83574c16e971dc67)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qtwindowsglobal.h

@@ -106,6 +106,7 @@ enum WindowsEventType // Simplify event types
     ExitSizeMoveEvent = WindowEventFlag + 23,
     PointerActivateWindowEvent = WindowEventFlag + 24,
     DpiScaledSizeEvent = WindowEventFlag + 25,
+    DpiChangedAfterParentEvent = WindowEventFlag + 27,
     MouseEvent = MouseEventFlag + 1,
     MouseWheelEvent = MouseEventFlag + 2,
     CursorEvent = MouseEventFlag + 3,
@@ -292,6 +293,8 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
         return HIWORD(wParamIn) ? QtWindows::AcceleratorCommandEvent : QtWindows::MenuCommandEvent;
     case WM_DPICHANGED:
         return QtWindows::DpiChangedEvent;
+    case WM_DPICHANGED_AFTERPARENT:
+        return QtWindows::DpiChangedAfterParentEvent;
     case WM_GETDPISCALEDSIZE:
         return QtWindows::DpiScaledSizeEvent;
     case WM_ENTERSIZEMOVE:

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1325,6 +1325,9 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
     case QtWindows::DpiChangedEvent:
         platformWindow->handleDpiChanged(hwnd, wParam, lParam);
         return true;
+    case QtWindows::DpiChangedAfterParentEvent:
+        platformWindow->handleDpiChangedAfterParent(hwnd);
+        return true;
 #if QT_CONFIG(sessionmanager)
     case QtWindows::QueryEndSessionApplicationEvent: {
         QWindowsSessionManager *sessionManager = platformSessionManager();

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -2007,6 +2007,14 @@ void QWindowsWindow::handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam)
     }
 }
 
+void QWindowsWindow::handleDpiChangedAfterParent(HWND hwnd)
+{
+    // FIXME: refactor, do we really need this?
+    setSavedDpi(GetDpiForWindow(hwnd));
+
+    checkForScreenChanged(QWindowsWindow::FromDpiChange);
+}
+
 static QRect normalFrameGeometry(HWND hwnd)
 {
     WINDOWPLACEMENT wp;
@@ -2150,12 +2158,14 @@ static inline bool equalDpi(const QDpi &d1, const QDpi &d2)
 
 void QWindowsWindow::checkForScreenChanged(ScreenChangeMode mode)
 {
-    if (parent() || QWindowsScreenManager::isSingleScreen())
+    if ((parent() && !parent()->isForeignWindow()) || QWindowsScreenManager::isSingleScreen())
         return;
 
     QPlatformScreen *currentScreen = screen();
+    auto topLevel = isTopLevel_sys() ? m_data.hwnd : GetAncestor(m_data.hwnd, GA_ROOT);
     const QWindowsScreen *newScreen =
-        QWindowsContext::instance()->screenManager().screenForHwnd(m_data.hwnd);
+        QWindowsContext::instance()->screenManager().screenForHwnd(topLevel);
+
     if (newScreen == nullptr || newScreen == currentScreen)
         return;
     // For screens with different DPI: postpone until WM_DPICHANGE

src/plugins/platforms/windows/qwindowswindow.h

@@ -286,6 +286,7 @@ public:
     void handleCompositionSettingsChanged();
     void handleDpiScaledSize(WPARAM wParam, LPARAM lParam, LRESULT *result);
     void handleDpiChanged(HWND hwnd, WPARAM wParam, LPARAM lParam);
+    void handleDpiChangedAfterParent(HWND hwnd);
 
     static void displayChanged();
     static void settingsChanged();