macOS: Interaction between ExtendClientAreaToDecorationsHint and SystemDecorations is broken

[grokys]

Describe the bug

When setting both ExtendClientAreaToDecorationsHint and SystemDecorations on a Window on macOS, the Window's visual state may be incorrect. There are 2 bugs I've found so far.

To Reproduce no.1
Steps to reproduce the behavior:

1. Edit Sandbox's MainWindow:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow"
        Title="Hello"
        ExtendClientAreaToDecorationsHint="True">
  <CheckBox Name="sysdec">System decorations</CheckBox>
</Window>

public MainWindow()
{
    this.InitializeComponent();
    this.AttachDevTools();

    var sysdec = this.GetControl<CheckBox>("sysdec");
    sysdec.IsChecked = SystemDecorations == SystemDecorations.Full;
    sysdec.IsCheckedChanged += (s, e) =>
        SystemDecorations = sysdec.IsChecked == true ? SystemDecorations.Full : SystemDecorations.None;
}

2. Show the window. Observe the no title bar is shown
3. Toggle the checkbox off and back on, such that SystemDecorations == Full (the original value)
4. Title bar is now displayed

To Reproduce no.2

1. Show a window with SystemDecorations="BorderOnly" and ExtendClientAreaToDecorationsHint="True"
2. The window will be shown with a disabled close button, which is incorrect

Expected behavior

For repro no.1: the title bar should not be visible once the checkbox is toggled
For repro no.2: The window buttons should all be hidden

Desktop (please complete the following information):

• OS: macOS
• Version: master and stable