Create a simple message box:

    QMessageBox msgBox;
    msgBox.setText("Hey yo!");
    msgBox.exec();

It will look like this:

Now add detailedText to the message box:

    QMessageBox msgBox;
    msgBox.setText("Hey yo!");
    msgBox.setDetailedText("Boring...");
    msgBox.exec();

Now it will look like this:

The "OK" button is gone. Pressing "Return" or "Escape" will show/hide the details.

Even when detailed text is available, the dialog should always have a button for properly closing it. By default, this should be the "OK" button. Pressing "Escape" should never show more details.

This is a regression as it works correctly in Qt 4.8.6 and 5.0.1.

This can be worked around by passing QMessageBox::Ok to the message box' ctor.

Update: This happens because calling QMessageBox::addButton removes the default button. Although this has already been the case in Qt 4.8.6, I think it is a bug because it is counter-intuitive and not even documented.