Window resizing area not working on MacOS

Steps to reproduce

1. Open gtk3-demo
2. Run any demo (Color Chooser for instance)
3. Try to resize the window with both left and right bottom corners

Current behavior

The cursor changes when the mouse is in the resizing area, but the window won't resize.

Expected outcome

Should be able to resize the window.

Version information

Gtk 3.24.29

On MacOS 10.15 and higher

Additional information

On MacOS, in the gdk quartz backend, the test_resize function uses a deprecated property of NSWindow, which is no more available since 10.15.

https://developer.apple.com/documentation/appkit/nswindow/1419531-showsresizeindicator?language=objc