Stop QMenuBar to steal keyboard focus when pressing Alt key in Linux.

interface/src/Application.cpp

@@ -803,6 +803,7 @@ bool Application::event(QEvent* event) {
 }
 
 void Application::keyPressEvent(QKeyEvent* event) {
+    qDebug("Application::keyPressEvent(%x)", event->key());
 
     _keysPressed.insert(event->key());
 
@@ -1053,6 +1054,7 @@ void Application::keyPressEvent(QKeyEvent* event) {
 }
 
 void Application::keyReleaseEvent(QKeyEvent* event) {
+    qDebug("Application::keyReleaseEvent(%x)", event->key());
 
     _keysPressed.remove(event->key());
 

interface/src/GLCanvas.cpp

@@ -23,6 +23,11 @@ GLCanvas::GLCanvas() : QGLWidget(QGLFormat(QGL::NoDepthBuffer)),
     _throttleRendering(false),
     _idleRenderInterval(MSECS_PER_FRAME_WHEN_THROTTLED)
 {
+#ifdef Q_OS_LINUX
+    // Cause GLCanvas::eventFilter to be called.
+    // It wouldn't hurt to do this on Mac and PC too; but apparently it's only needed on linux.
+    qApp->installEventFilter(this);
+#endif
 }
 
 bool GLCanvas::isThrottleRendering() const {
@@ -162,3 +167,34 @@ void GLCanvas::dragEnterEvent(QDragEnterEvent* event) {
 void GLCanvas::dropEvent(QDropEvent* event) {
     Application::getInstance()->dropEvent(event);
 }
+
+// Pressing Alt (and Meta) key alone activates the menubar because its style inherits the
+// SHMenuBarAltKeyNavigation from QWindowsStyle. This makes it impossible for a scripts to
+// receive keyPress events for the Alt (and Meta) key in a reliable manner.
+//
+// This filter catches events before QMenuBar can steal the keyboard focus.
+// The idea was borrowed from
+// http://www.archivum.info/qt-interest@trolltech.com/2006-09/00053/Re-(Qt4)-Alt-key-focus-QMenuBar-(solved).html
+
+bool GLCanvas::eventFilter(QObject*, QEvent* event) {
+    switch (event->type()) {
+        case QEvent::KeyPress:
+        case QEvent::KeyRelease:
+        case QEvent::ShortcutOverride:
+        {
+            QKeyEvent* keyEvent = static_cast<QKeyEvent*>(event);
+            if (keyEvent->key() == Qt::Key_Alt || keyEvent->key() == Qt::Key_Meta) {
+                if (event->type() == QEvent::KeyPress)
+                    keyPressEvent(keyEvent);
+                else if (event->type() == QEvent::KeyRelease)
+                    keyReleaseEvent(keyEvent);
+                else
+                    QGLWidget::event(event);
+                return true;
+            }
+        }
+        default:
+            break;
+    }
+    return false;
+}

interface/src/GLCanvas.h

@@ -50,6 +50,7 @@ protected:
 private slots:
     void activeChanged(Qt::ApplicationState state);
     void throttleRender();
+    bool eventFilter(QObject*, QEvent* event);
 };
 
 #endif // hifi_GLCanvas_h