Stylesheets affect QMessageBox without Options.DontUseNativeDialog

As of Qt 6.6 but going through all 6.7.* versions a QMessageBox on macOS will not display a native Cocoa message box if the message box would be affected by a stylesheet, completely ignoring Options.DontUseNativeDialog.

This effectively removes the ability to have an application-wide stylesheet while still using the native macOS Cocoa message boxes as anything that would in theory affect QMessageBox - like a font or border setting on QWidget - will force a non-native QMessageBox.

It also clashes with the behaviour of QFileDialog which still defaults to a Cocoa dialog regardless if it would be affected by an application-wide stylesheet (it behaves as expected and actually respects the Option.DontUseNativeDialog / the absence of that option).

The source of the problem seems to in qmessagebox.cpp / QMessageBoxPrivate::canBeNative(): https://github.com/qt/qtbase/blob/4ed994487484b37ff960144806445f8f9ce42286/src/widgets/dialogs/qmessagebox.cpp#L2854

The current check will force a non-native dialog whenever a stylesheet is present, regardless if either of the two DontUseNativeDialogs options is present:

    if (QCoreApplication::testAttribute(Qt::AA_DontUseNativeDialogs)
        || q->testAttribute(Qt::WA_DontShowOnScreen)
        || q->testAttribute(Qt::WA_StyleSheet)
        || (options->options() & QMessageDialogOptions::Option::DontUseNativeDialog)) {
        return false;
    }