xcb: Propagate size hints when window changes its scaling factor

Properly scale size hints to the new scaling factor.

Fixes: QTBUG-80476
Change-Id: I1081c9b01560f7e434f0f1de0199ef3d3cae787c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1398,6 +1398,8 @@ void QXcbWindow::propagateSizeHints()
     }
 
     xcb_icccm_set_wm_normal_hints(xcb_connection(), m_window, &hints);
+
+    m_sizeHintsScaleFactor = QHighDpiScaling::scaleAndOrigin(screen()).factor;
 }
 
 void QXcbWindow::requestActivateWindow()
@@ -1789,6 +1791,9 @@ void QXcbWindow::handleConfigureNotifyEvent(const xcb_configure_notify_event_t *
     // will make the comparison later.
     QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
 
+    if (!qFuzzyCompare(QHighDpiScaling::scaleAndOrigin(newScreen).factor, m_sizeHintsScaleFactor))
+        propagateSizeHints();
+
     // Send the synthetic expose event on resize only when the window is shrinked,
     // because the "XCB_GRAVITY_NORTH_WEST" flag doesn't send it automatically.
     if (!m_oldWindowSize.isEmpty()

src/plugins/platforms/xcb/qxcbwindow.h

@@ -280,6 +280,8 @@ protected:
 
     QXcbSyncWindowRequest *m_pendingSyncRequest = nullptr;
     int m_swapInterval = -1;
+
+    qreal m_sizeHintsScaleFactor = 1.0;
 };
 
 class QXcbForeignWindow : public QXcbWindow