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>
Pick-to: 6.4
Task-number: QTBUG-103383
Change-Id: Icf85dd0afa806609dbbe0ffc36efbc5127962c39
Reviewed-by: <stefan.wastl@native-instruments.de>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/plugins/platforms/windows/qwindowswindow.cpp b/src/plugins/platforms/windows/qwindowswindow.cpp
index 61d512a33f..c953abbc61 100644
--- a/src/plugins/platforms/windows/qwindowswindow.cpp
+++ b/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