How to display a tab widget's corner widget on the left/right side

I have a QTabWidget with its tabs on the West, then I tried to add a corner widget to it, but it did not appear. If I set the tabs position to North or South, the corner widget gets displayed, but not on the side.

QWidget *w = new QWidget();
QTabWidget *t = new QTabWidget(w);

w->setMinimumSize(800,600);

t->addTab(new QWidget(),"Tab");
t->addTab(new QWidget(),"Tab");
t->addTab(new QWidget(),"Tab");
t->addTab(new QWidget(),"Tab");

t->setTabPosition(QTabWidget::TabPosition::East);

t->setGeometry(100,100,400,400);

QPushButton *button = new QPushButton("Button");

//for debugging purposes
button->setObjectName("ss");

//for debugging purposes
t->setStyleSheet("background: red");
button->setStyleSheet("background: blue;");

//for debugging purposes
t->setCornerWidget(button1,Qt::TopLeftCorner);
//t->setCornerWidget(button1,Qt::TopRightCorner);
//t->setCornerWidget(button1,Qt::BottomLeftCorner);
//t->setCornerWidget(button1,Qt::BottomRightCorner);
    
w->show();

//for debugging purposes
//qDebug()<<button->geometry();
//button->setGeometry(0,0,50,50);

//for debugging purposes
button->connect(button,&QPushButton::clicked,[](){qDebug()<<"corner widget click?";});

I tried to use all 4 corners, and right corners have no effect, but the left causes an empty space before the tabs, here's how it looks, the empty space is at the top right corner:

I tried to set the stylesheet of the button I'm using as a corner widget, so that it might appear if hidden, but resulted in nothing.

I checked the button's geometry, and it got QRect(0,0 0x0), and that is after using show(). So I tried to set its geometry, but resulted in nothing as well (but the geometry got updated correctly), the button is still not displayed.

I also tried to check for the button whereabouts by connecting its clicked() signal to a qDebug(), I clicked all over the widget and got no output.

QTabWidget::setCornerWidget:

Note: Corner widgets are designed for North and South tab positions; other orientations are known to not work properly.

I get from that, that it's not entirely impossible to get it to work.

From the Qt documentation (qt6), QTabWidget::setCornerWidget: says:

Note: Corner widgets are designed for North and South tab positions; other orientations are known to not work properly.

That means they do work, but are messy to and inconvenient to use.

Here's a way to use QTabWidget corner widget on the side:

As already noted in the question, setting a corner widget while tabs position is West or East, causes a small gap before the tabs without anything appearing there.

But if you set QTabWidget's corner widget minimum size, it will appear, which solves a problem but causes another, because now I need to calculate that size myself, or make room for my corner widget.

QTabWidget *t = new QTabWidget();

//I needed the stacked widget so I can use its geometry to calculate the empty corner size
QStackedWidget *stack_widget = t->findChild<QStackedWidget*>("qt_tabwidget_stackedwidget");
t->setMinimumSize(800,600);
t->addTab(new QWidget(),"Tab1");
t->addTab(new QWidget(),"Tab2");
t->addTab(new QWidget(),"Tab3");
t->addTab(new QWidget(),"Tab4");
t->setTabPosition(QTabWidget::TabPosition::West);

QToolButton *button1 = new QToolButton();
button1->setIcon(QIcon(":/icons/collapse.png"));

t->setCornerWidget(button1,Qt::TopLeftCorner);

t->show();
    
//width is equal to where the stack widget starts (x coordinate)
//height is equal to where the tab bar starts (y coordinate)
//I subtracted 1 from stackwidget's x because it simply looked better
t->cornerWidget(Qt::TopLeftCorner)->setMinimumSize(stack_widget->geometry().x()-1,
                                                   t->tabBar()->geometry().y());

If you need more space for the corner widget, you'll need to move the tab bar, because corner widget will cover it, you can do that using stylesheets.

See: Qt Style Sheets Examples: Customizing QTabWidget and QTabBar.

Example:

t->setStyleSheet("QTabWidget::tab-bar "
                 "{"
                     "top: 50px;" /* push down by 50px */
                 "}");

Here's how it looks with that being the only addition and change to my MRE:

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Suggestion: QTabWidget::paintEvent might be a better solution.