In my case, I create QWidget containing QWebEngineView. But when I set the main window as the parent of the QWebEngineView with the following code snippet, the main window will be resized.

TestWidget::TestWidget(QWidget* parent)  // parent is main window
{
    QHBoxLayout *layout = new QHBoxLayout(parent);
    QWebEngineView *view = new QWebEngineView;
    view->createWinId();
    view->setParent(parent);
    layout->addWidget(view); 
}

Before calling setParent(), the main window is maximum. After calling, the main window will be resized.

This problem can be reproduced with the attached sample code. The reproduce steps:
1. Run the application.

2. Click "QWebEngineView" in toolbar.

Result: Before click, the window is maximum. After click, the window is not maximum any more.