Hiding widgets in a QMainWindow layout with a translucent background

I got QMainWindow with WA_TranslucentBackground. I want to hide and show some elements in its layout. It looks like the layout or background doesn't repaint after hiding the widget.

Before hiding btn2:

After hiding btn2:

I made a sample to demonstrate this:

MainWindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include "QtWidgets/qboxlayout.h"
#include "QtWidgets/qpushbutton.h"
#include "ui_mainwindow.h"
#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui

{ class MainWindow; }

QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
Q_OBJECT
QWidget *centralWidget;
QVBoxLayout *btnsLayout;
QPushButton *btn1;
QPushButton *btn2;

Ui::MainWindow *ui;

public:
MainWindow(QWidget *parent = nullptr)
: QMainWindow(parent)
, ui(new Ui::MainWindow)

{ ui->setupUi(this); setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window); setAttribute(Qt::WA_NoSystemBackground, true); setAttribute(Qt::WA_TranslucentBackground, true); btn1 = new QPushButton("btn1", this); btn2 = new QPushButton("btn2", this); btnsLayout = new QVBoxLayout; btnsLayout->addWidget(btn1); btnsLayout->addWidget(btn2); connect(btn1, &QPushButton::clicked, this, &MainWindow::onBtnClick); centralWidget = new QWidget(this); centralWidget->setLayout(btnsLayout); setCentralWidget(centralWidget); }

~MainWindow() { delete ui; }

public slots:
void onBtnClick(bool)

{ btn2->hide(); // works! // setVisible(false); // setVisible(true); // or       // resize(width() + 1, height()); }

};
#endif // MAINWINDOW_H
I'm ok with the layout moving btn1, but I need to get rid of the "shadows" of the buttons.

I tried removeWidget(btn2), delete btn2, btnsLayout->update(),  MainWindow::update(), and repaint(), nothing helps.

The only workaround I found is to hide and show or resize the entire main window (commented out part), but I think there's gotta be a better way.