fullscreen: rebuild & reconnect menu only on desktops with global menu

[ronso0]

follow-up for #11328:
keep the menubar on all Linux desktops that don't use global menubars to avoid pointless operations.

[daschuer]

For my understanding Qt::AA_DontUseNativeMenuBar is always false.

This change has been introduced in Qt 5.7.1 (Ubuntu Bionic has 5.9):
qt/qtbase@8cd28ea

According to the comment we can check whether the menu bar is already non-native before chaining to full screen using MenuBar::isNativeMenuBar().

[daschuer]

Is there a Linux distro that has no native menu bar (DBus)?
If not we can just close this?

[ronso0]

Oh, you got me there, I didn't investigat thoroughly.

What I'm after is actually the unchangeable 'supports global menu' property, like qt5ct reports it when Mixxx is started.
I changed the approach and adopted the way qt5ct detects it:
https://github.com/desktop-app/qt5ct/blob/9f60cd2352a4dcc55c8ca267f29bd8fff5c6a659/src/qt5ct-qtplugin/qt5ctplatformtheme.cpp#L102-L105

QDBusConnection conn = QDBusConnection::sessionBus();
m_dbusGlobalMenuAvailable = conn.interface()->isServiceRegistered("com.canonical.AppMenu.Registrar");
qCDebug(lqt5ct) << "D-Bus global menu:" << (m_dbusGlobalMenuAvailable ? "yes" : "no");

Is this okay in your opinion?

[daschuer]

The Qt Implementation has also a null check.
https://github.com/qt/qtbase/blob/6e2963200642fa0171b247cb986726ca124795b0/src/gui/platform/unix/qgenericunixthemes.cpp#L108

[daschuer]

For completeness you may also check the QT_NO_DBUS macro.

[daschuer]

Is this okay in your opinion?

Yes.

[ronso0]

Yep, implemented as you suggested. Thanks for you help!

[daschuer]

LGTM, Thank you.