Update qt_mainwindow.cpp

This commit fixes issue #3485

src/qt/qt_mainwindow.cpp

@@ -615,6 +615,15 @@ MainWindow::MainWindow(QWidget *parent)
     if (!vnc_enabled)
         video_setblit(qt_blit);
 
+        if (start_in_fullscreen) {
+        connect(ui->stackedWidget, &RendererStack::blit, this, [this] () {
+            if (start_in_fullscreen) {
+                QTimer::singleShot(100, ui->actionFullscreen, &QAction::trigger);
+                start_in_fullscreen = 0;
+            }
+        });
+    }
+
 #ifdef MTR_ENABLED
     {
         ui->actionBegin_trace->setVisible(true);
@@ -841,10 +850,6 @@ MainWindow::showEvent(QShowEvent *event)
         QApplication::processEvents();
         this->adjustSize();
     }
-    if (start_in_fullscreen) {
-        start_in_fullscreen = 0;
-        QTimer::singleShot(0, ui->actionFullscreen, &QAction::trigger);
-    }
 }
 
 void