Qt: can create widget in ctor, but not in mousePressEvent

Question

I want to dynamically create a child widget on the click of the mouse. When I manually create it in ctor, everything is ok.

Foo::Foo(QWidget *partent) : QWidget(parent)
{
    auto *txt{ new QPlainTextEdit(this) };
}

but when I do the same in mousePressEvent, it doesn't appear.

Foo::mousePressEvent(QMouseEvent *event)
{
    auto *txt{ new QPlainTextEdit(this) };

    QWidget::mousePressEvent(event);
}

What might be the problem? Is there something in the ctor that triggers the appearance of the widget?

UPD: adding the full code, as asked in the comments.

// foo.h

#include <QMouseEvent>
#include <QWidget>

class Foo : public QWidget
{
    Q_OBJECT

public:
    explicit Foo(QWidget *parent = nullptr);

protected:
    void mousePressEvent(QMouseEvent *event) override;
}

// main.cpp

#include "foo.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Foo w;
    w.show();
    return a.exec();
}

Answer 1

As I understand from your question , I write this example for you :

My example has MainWindow , and in

mainwindow.h

#include <QMainWindow>
#include <QPlainTextEdit>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();

private:
Ui::MainWindow *ui;

// QWidget interface
protected:
void mousePressEvent(QMouseEvent *event);


private:
QPlainTextEdit *_text;
};

and in mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    _text = new QPlainTextEdit(this);
    _text->setGeometry(event->x(),event->y(),_text->width(),_text->height());
    _text->setPlainText("Hello World");
    _text->show();

}

Anywhere on the Window , when you press the mouse one QPlainTextEdit whit Hello World text appears.

the out put is :