QTableView::cornerButtonEnabled says:

[...]Clicking on this button will select all the cells in the table view.

But using Qt 6.7.0, that leads to confusion when clicking that button, and finding out that it does not select the whole table, and deselects all selected rows.

From Qt 6.7.0 source code:

cornerWidgetConnection = QObject::connect(
          cornerWidget, &QTableCornerButton::clicked,
          q, &QTableView::reset);

The latest version I could find with the actual documented behavior is 6.6.3:

QObject::connect(cornerWidget, SIGNAL(clicked()), q, SLOT(selectAll()));