fullscreen: workaround window manager consuming fullscreen hotkey

the window manager (xfwm4) hotkey and the Mixxx hotkey are identical, and the keystroke
is consumed by the window manager thus it doesn't reach Mixxx.
Catch the QWindowStateChangeEvent and emit fullScreenChanged(isFullScreen()) manually
to toggle the Fullscreen checkbox and hide the menubar when going fullscreen.

src/mixxxmainwindow.cpp

@@ -1080,6 +1080,25 @@ bool MixxxMainWindow::eventFilter(QObject* obj, QEvent* event) {
                 return true;
             }
         }
+#ifdef __LINUX__
+    } else if (event->type() == QEvent::WindowStateChange) {
+        // detect if we entered or quit fullscreen mode
+        QWindowStateChangeEvent* changeEvent =
+                static_cast<QWindowStateChangeEvent*>(event);
+        bool wasFullScreen = changeEvent->oldState() == Qt::WindowFullScreen;
+        if ((isFullScreen() && !wasFullScreen) ||
+                (!isFullScreen() && wasFullScreen)) {
+            if (isFullScreen() && !wasFullScreen) {
+                qWarning() << "     WindowStateChange: enter fullscreen";
+            } else {
+                qWarning() << "     WindowStateChange: leave fullscreen";
+            }
+            qWarning() << "     emit fullScreenChanged(" << isFullScreen() << ")";
+            // this will toggle the Fullscreen checkbox and hide the menubar if
+            // we go fullscreen
+            emit fullScreenChanged(isFullScreen());
+        }
+#endif
     }
     // standard event processing
     return QMainWindow::eventFilter(obj, event);

src/widget/wmainmenubar.cpp

@@ -303,15 +303,28 @@ void WMainMenuBar::initialize() {
     shortcuts << QKeySequence("F11");
 #endif
     QKeySequence osShortcut = QKeySequence::FullScreen;
+    qWarning() << "##";
+    qWarning() << "## fullscreen OS hotkey:" << osShortcut.toString();
+    qWarning() << "## osShortcut.isEmpty:" << osShortcut.isEmpty();
+    // Funnily this returns 1 = QKeySequence::PartialMatch even though
+    // KeySequence::FullScreen is empty
+    qWarning() << "## osShortcut == F11:" << osShortcut.matches(shortcuts.first());
     // Note(ronso0) Only add the OS shortcut if it's not empty and not F11.
     // In some Linux distros the window managers doesn't pass the OS fullscreen
     // key sequence to Mixxx for some reason.
     // Both adding an empty key sequence or the same sequence twice can render
     // the fullscreen shortcut nonfunctional.
     // https://bugs.launchpad.net/mixxx/+bug/1882474  PR #3011
     if (!osShortcut.isEmpty() && !shortcuts.contains(osShortcut)) {
+        qWarning() << "## add OS hotkey";
         shortcuts << osShortcut;
+        int i = 1;
+        for (auto ks : shortcuts) {
+            qWarning() << "##" << i << ks.toString();
+            i++;
+        }
     }
+    qWarning() << "##";
 
     pViewFullScreen->setShortcuts(shortcuts);
     pViewFullScreen->setShortcutContext(Qt::ApplicationShortcut);