Modal QMessage always displays a zoom button on macOS (regression 5.9.3 -> 5.10.0)

On macOS with Qt 5.10.0, a QMessage dialog always displays the green zoom button - even if you explicitly tells it not to display such button. For modal alerts this is a really bad user experience as it makes no sense to zoom a small alert to take the full screen. Attached is a sample project 'QMessageBoxBug' to reproduce the issue.

When compiling the sources with Qt 5.9.3, you will see such an alert on macOS 10.13.2:

If you compile the source code with Qt 5.10.0, you will see an alert with the green zoom button:

The complete project source code is attached QMessageBoxBug.zip and the bug can be reproduced with 5 lines of code:

QMessageBox* messageBox = new QMessageBox("My App", "Text", QMessageBox::Question, QMessageBox::Yes | QMessageBox::Default, QMessageBox::NoButton, QMessageBox::NoButton);
messageBox->setInformativeText("Informative text");
messageBox->setButtonText(QMessageBox::Yes, "OK");
messageBox->setWindowFlags(Qt::Dialog | Qt::CustomizeWindowHint | Qt::WindowTitleHint);
messageBox->exec();

Steps de reproduce:

1. Download the attached QMessageBoxBug project or copy the 5 lines of code in any project
2. Compile on macOS 10.13.2 using Qt 5.10.0
3. Run the application

Result: The QMessageBox alert has a green zoom button.

Expected result: The QMessageBox alert should not have a green zoom button

Notes:

• This issue is not reproducible with Qt 5.9.3
• So far I did not find any workaround
• Also note that the close and minimize button are disabled but visible with Qt 5.10.0. With Qt 5.9.3 they don't appear at all as expected.