I have a problem with Qt 5.12.0 Beta 3 with checkable MenuItems on MacOS Mojave:
(Unable to reproduce this with Qt 5.11.1)

ApplicationWindow {
    id: window
    visible: true
    width: 1280; height: 720

    menuBar: MenuBar {
        Menu {
            title: "MainMenu"
            MenuItem { text: "Cut"; checkable: true }
            MenuItem { text: "Copy"; checkable: true }
            MenuItem { text: "Paste"; checkable: true}
        }
    }
}

 When opening the menu and clicking either MenuItem, the MainMenu popup closes, but the button remains "active", though closed. You have to click the button twice to open the menu again ("hidden close click" + click to open again).

Either the popup should remain open (preferable actually in my case, but this is maybe a separate feature request), or the button needs to be non-active.

See attached screenshots.