Windows QPA: Fix resize loops when moving fixed size windows between screens

Postpone the screen change until the DPI changed event in case a move
between screens with different DPI is detected.

Task-number: QTBUG-65580
Change-Id: I356f144b243d7d1ce7feabf0434c3f534b903965
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Andre de la Rocha <andre.rocha@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1322,17 +1322,24 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
 #endif
     }   break;
     case QtWindows::DpiChangedEvent: {
-        if (!resizeOnDpiChanged(platformWindow->window()))
-            return false;
-        platformWindow->setFlag(QWindowsWindow::WithinDpiChanged);
-        const RECT *prcNewWindow = reinterpret_cast<RECT *>(lParam);
-        qCDebug(lcQpaWindows) << __FUNCTION__ << "WM_DPICHANGED"
-            << platformWindow->window() << *prcNewWindow;
-        SetWindowPos(hwnd, nullptr, prcNewWindow->left, prcNewWindow->top,
-                     prcNewWindow->right - prcNewWindow->left,
-                     prcNewWindow->bottom - prcNewWindow->top, SWP_NOZORDER | SWP_NOACTIVATE);
-        platformWindow->clearFlag(QWindowsWindow::WithinDpiChanged);
-        return true;
+        // Try to apply the suggested size first and then notify ScreenChanged
+        // so that the resize event sent from QGuiApplication incorporates it
+        // WM_DPICHANGED is sent with a size that avoids resize loops (by
+        // snapping back to the previous screen, see QTBUG-65580).
+        const bool doResize = resizeOnDpiChanged(platformWindow->window());
+        if (doResize) {
+            platformWindow->setFlag(QWindowsWindow::WithinDpiChanged);
+            platformWindow->updateFullFrameMargins();
+            const auto prcNewWindow = reinterpret_cast<RECT *>(lParam);
+            qCDebug(lcQpaWindows) << __FUNCTION__ << "WM_DPICHANGED"
+                << platformWindow->window() << *prcNewWindow;
+            SetWindowPos(hwnd, nullptr, prcNewWindow->left, prcNewWindow->top,
+                         prcNewWindow->right - prcNewWindow->left,
+                         prcNewWindow->bottom - prcNewWindow->top, SWP_NOZORDER | SWP_NOACTIVATE);
+            platformWindow->clearFlag(QWindowsWindow::WithinDpiChanged);
+        }
+        platformWindow->checkForScreenChanged(QWindowsWindow::FromDpiChange);
+        return doResize;
     }
 #if QT_CONFIG(sessionmanager)
     case QtWindows::QueryEndSessionApplicationEvent: {

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1861,28 +1861,41 @@ void QWindowsWindow::handleResized(int wParam)
     }
 }
 
-void QWindowsWindow::checkForScreenChanged()
+static inline bool equalDpi(const QDpi &d1, const QDpi &d2)
 {
-    if (parent())
+    return qFuzzyCompare(d1.first, d2.first) && qFuzzyCompare(d1.second, d2.second);
+}
+
+void QWindowsWindow::checkForScreenChanged(ScreenChangeMode mode)
+{
+    if (parent() || QWindowsScreenManager::isSingleScreen())
         return;
 
     QPlatformScreen *currentScreen = screen();
-    const auto &screenManager = QWindowsContext::instance()->screenManager();
-    const QWindowsScreen *newScreen = screenManager.screenForHwnd(m_data.hwnd);
-    if (newScreen != nullptr && newScreen != currentScreen) {
-        qCDebug(lcQpaWindows).noquote().nospace() << __FUNCTION__
-            << ' ' << window() << " \"" << currentScreen->name()
-            << "\"->\"" << newScreen->name() << '"';
-        updateFullFrameMargins();
-        setFlag(SynchronousGeometryChangeEvent);
-        QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
+    const QWindowsScreen *newScreen =
+        QWindowsContext::instance()->screenManager().screenForHwnd(m_data.hwnd);
+    if (newScreen == nullptr || newScreen == currentScreen)
+        return;
+    // For screens with different DPI: postpone until WM_DPICHANGE
+    if (mode == FromGeometryChange
+        && !equalDpi(currentScreen->logicalDpi(), newScreen->logicalDpi())) {
+        return;
     }
+    qCDebug(lcQpaWindows).noquote().nospace() << __FUNCTION__
+        << ' ' << window() << " \"" << currentScreen->name()
+        << "\"->\"" << newScreen->name() << '"';
+    if (mode == FromGeometryChange)
+        setFlag(SynchronousGeometryChangeEvent);
+    updateFullFrameMargins();
+    QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
 }
 
 void QWindowsWindow::handleGeometryChange()
 {
     const QRect previousGeometry = m_data.geometry;
     m_data.geometry = geometry_sys();
+    if (testFlag(WithinDpiChanged))
+        return;  // QGuiApplication will send resize
     QWindowSystemInterface::handleGeometryChange(window(), m_data.geometry);
     // QTBUG-32121: OpenGL/normal windows (with exception of ANGLE) do not receive
     // expose events when shrinking, synthesize.

src/plugins/platforms/windows/qwindowswindow.h

@@ -334,7 +334,8 @@ public:
     void alertWindow(int durationMs = 0);
     void stopAlertWindow();
 
-    void checkForScreenChanged();
+    enum ScreenChangeMode { FromGeometryChange, FromDpiChange };
+    void checkForScreenChanged(ScreenChangeMode mode = FromGeometryChange);
 
     static void setTouchWindowTouchTypeStatic(QWindow *window, QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes);
     void registerTouchWindow(QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes = QWindowsWindowFunctions::NormalTouch);