kcm/edges: Draw the ratio of the monitor we are on

At the moment it's showing whatever is QGuiApplication::primaryScreen(),
which on Wayland is random.

src/kcmkwin/kwinscreenedges/monitor.cpp

@@ -53,8 +53,6 @@ static QScreen *screenFromWidget(const QWidget *widget)
 Monitor::Monitor(QWidget *parent)
     : ScreenPreviewWidget(parent)
 {
-    QRect avail = screenFromWidget(this)->geometry();
-    setRatio((qreal)avail.width() / (qreal)avail.height());
     for (int i = 0;
          i < 8;
          ++i) {
@@ -97,6 +95,17 @@ void Monitor::resizeEvent(QResizeEvent *e)
     checkSize();
 }
 
+bool Monitor::event(QEvent *event)
+{
+    const bool r = ScreenPreviewWidget::event(event);
+    if (event->type() == QEvent::ScreenChangeInternal) {
+        QRect avail = screenFromWidget(this)->geometry();
+        setRatio((qreal)avail.width() / (qreal)avail.height());
+        checkSize();
+    }
+    return r;
+}
+
 void Monitor::checkSize()
 {
     QRect contentsRect = previewRect();

src/kcmkwin/kwinscreenedges/monitor.h

@@ -65,6 +65,7 @@ Q_SIGNALS:
 
 protected:
     void resizeEvent(QResizeEvent *e) override;
+    bool event(QEvent *event) override;
 
 private:
     class Corner;