effects/magnifier: Schedule a repaint when window is damaged

This is needed to make per-screen rendering work correctly.

BUG: 431652

effects/magnifier/magnifier.cpp

@@ -58,6 +58,7 @@ MagnifierEffect::MagnifierEffect()
     effects->registerGlobalShortcut(Qt::META + Qt::Key_0, a);
 
     connect(effects, &EffectsHandler::mouseChanged, this, &MagnifierEffect::slotMouseChanged);
+    connect(effects, &EffectsHandler::windowDamaged, this, &MagnifierEffect::slotWindowDamaged);
 
     reconfigure(ReconfigureAll);
 }
@@ -332,6 +333,13 @@ void MagnifierEffect::slotMouseChanged(const QPoint& pos, const QPoint& old,
         effects->addRepaintFull();
 }
 
+void MagnifierEffect::slotWindowDamaged()
+{
+    if (isActive()) {
+        effects->addRepaint(magnifierArea());
+    }
+}
+
 bool MagnifierEffect::isActive() const
 {
     return zoom != 1.0 || zoom != target_zoom;

effects/magnifier/magnifier.h

@@ -50,6 +50,7 @@ private Q_SLOTS:
     void slotMouseChanged(const QPoint& pos, const QPoint& old,
                               Qt::MouseButtons buttons, Qt::MouseButtons oldbuttons,
                               Qt::KeyboardModifiers modifiers, Qt::KeyboardModifiers oldmodifiers);
+    void slotWindowDamaged();
     void destroyPixmap();
 private:
     QRect magnifierArea(QPoint pos = cursorPos()) const;