Resize the window when the screen's dpi changes

That's what the MSDN documentation say one should do in the handler
for WM_DPICHANGED

[ChangeLog][QtGui][Windows] Windows are now automatically resized
when they are moved on a screen to adapt to the new pixel ratio.

Task-number: QTBUG-55510
Task-number: QTBUG-48242
Change-Id: I7688f12165f76585d75686e2e94b0fc562627be2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/platforms/windows/qtwindowsglobal.h

@@ -56,6 +56,10 @@
 #  define WM_GESTURE 0x0119
 #endif
 
+#ifndef WM_DPICHANGED
+#  define WM_DPICHANGED 0x02E0
+#endif
+
 QT_BEGIN_NAMESPACE
 
 namespace QtWindows
@@ -96,6 +100,7 @@ enum WindowsEventType // Simplify event types
     FocusInEvent = WindowEventFlag + 17,
     FocusOutEvent = WindowEventFlag + 18,
     WhatsThisEvent = WindowEventFlag + 19,
+    DpiChangedEvent = WindowEventFlag + 21,
     MouseEvent = MouseEventFlag + 1,
     MouseWheelEvent = MouseEventFlag + 2,
     CursorEvent = MouseEventFlag + 3,
@@ -266,6 +271,8 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
 #endif
     case WM_GESTURE:
         return QtWindows::GestureEvent;
+    case WM_DPICHANGED:
+        return QtWindows::DpiChangedEvent;
     default:
         break;
     }

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1088,6 +1088,15 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
         return true;
 #endif
     }   break;
+    case QtWindows::DpiChangedEvent: {
+        platformWindow->setFlag(QWindowsWindow::WithinDpiChanged);
+        const RECT *prcNewWindow = reinterpret_cast<RECT *>(lParam);
+        SetWindowPos(hwnd, NULL, prcNewWindow->left, prcNewWindow->top,
+                     prcNewWindow->right - prcNewWindow->left,
+                     prcNewWindow->bottom - prcNewWindow->top, SWP_NOZORDER | SWP_NOACTIVATE);
+        platformWindow->clearFlag(QWindowsWindow::WithinDpiChanged);
+        return true;
+    }
 #if !defined(QT_NO_SESSIONMANAGER)
     case QtWindows::QueryEndSessionApplicationEvent: {
         QWindowsSessionManager *sessionManager = platformSessionManager();

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -2114,8 +2114,12 @@ void QWindowsWindow::setFrameStrutEventsEnabled(bool enabled)
 
 void QWindowsWindow::getSizeHints(MINMAXINFO *mmi) const
 {
+    // We don't apply the min/max size hint as we change the dpi, because we did not adjust the
+    // QScreen of the window yet so we don't have the min/max with the right ratio
+    if (!testFlag(QWindowsWindow::WithinDpiChanged)) {
         const QWindowsGeometryHint hint(window(), m_data.customMargins);
         hint.applyToMinMaxInfo(m_data.hwnd, mmi);
+    }
 
     if ((testFlag(WithinMaximize) || (window()->windowState() == Qt::WindowMinimized))
             && (m_data.flags & Qt::FramelessWindowHint)) {

src/plugins/platforms/windows/qwindowswindow.h

@@ -205,7 +205,8 @@ public:
         MaximizeToFullScreen = 0x80000,
         InputMethodDisabled = 0x100000,
         Compositing = 0x200000,
-        HasBorderInFullScreen = 0x400000
+        HasBorderInFullScreen = 0x400000,
+        WithinDpiChanged = 0x800000,
     };
 
     QWindowsWindow(QWindow *window, const QWindowsWindowData &data);