Mac(/bug?) Control + Tab shortcut doesn't do anything

[René J.V. Bertin]

Hello,

I just discovered that the Control+Tab and Control+Shift+Tab shortcuts don't work in KF5 applications on Mac:

- In a pure Qt application I can assign Control+Tab to an action (using Qt::Meta+Qt::Key_Tab, evidently) and pressing the key combination will trigger the action.
- In KF5, I can use for installed KActionCollection::setDefaultShortcut() to define the Control+Tab shortcut; it will show up but never trigger
- trying to define it via the KXmlGui shortcut editor shows the same behaviour: it's like pressing the key combination is not detected.

Qt is capable of detecting it, so one explanation would be that somewhere in KF5 code there is a filter on Qt::Meta+Qt::Key_Tab (because Command+Tab is a reserved shortcut bringing up the app switcher?). If that's the case, it should filter out Qt::CTRL+Qt::Key_Tab, but I cannot find any such filter.

FWIW, the shortcut works just fine when I use Qt's XCB QPA plugin on Mac.

Thanks,
René