[wayland] Use a dummy window for QtWayland's popup window handling

QtWayland only creates popup windows if they have a parent QWindow or
if there is any window which had input. It's not enough to fake an
enter, it needs to be either a pointer button press or key press.

As KWin's useraction menu doesn't have a parent and we most likely
never send a pointer press to any QWindow it doesn't get shown. To
circumvent this we create a dummy window and fake a button press/release
on the window. After that Qt is tricked into believing there's a parent
window and shows the popup.

Faking the input is only done with at least Qt 5.5 as QtWayland crashes
on pointer event without a keymap being installed. As KWin does not yet
send keymaps we better disable the dangerous code path. With Qt 5.5 the
crash condition is fixed.

main_wayland.cpp

@@ -221,6 +221,7 @@ void ApplicationWayland::continueStartupWithX()
             Xcb::sync(); // Trigger possible errors, there's still a chance to abort
 
             notifyKSplash();
+            waylandServer()->createDummyQtWindow();
         }
     );
     eglInitWatcher->setFuture(QtConcurrent::run([] {

wayland_server.cpp

@@ -27,6 +27,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 // Client
 #include <KWayland/Client/connection_thread.h>
 #include <KWayland/Client/registry.h>
+#include <KWayland/Client/surface.h>
 // Server
 #include <KWayland/Server/compositor_interface.h>
 #include <KWayland/Server/datadevicemanager_interface.h>
@@ -35,6 +36,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <KWayland/Server/seat_interface.h>
 #include <KWayland/Server/shell_interface.h>
 
+// Qt
+#include <QWindow>
+
 // system
 #include <sys/types.h>
 #include <sys/socket.h>
@@ -94,6 +98,13 @@ void WaylandServer::init(const QByteArray &socketName)
                 // skip Xwayland clients, those are created using standard X11 way
                 return;
             }
+            if (surface->client() == m_qtConnection) {
+                // one of Qt's windows
+                if (m_dummyWindowSurface && (m_dummyWindowSurface->id() == surface->surface()->id())) {
+                    fakeDummyQtWindowInput();
+                    return;
+                }
+            }
             auto client = new ShellClient(surface);
             if (auto c = Compositor::self()) {
                 connect(client, &Toplevel::needsRepaint, c, &Compositor::scheduleRepaint);
@@ -194,4 +205,37 @@ void WaylandServer::removeClient(ShellClient *c)
     emit shellClientRemoved(c);
 }
 
+void WaylandServer::createDummyQtWindow()
+{
+    if (m_dummyWindow) {
+        return;
+    }
+    m_dummyWindow.reset(new QWindow());
+    m_dummyWindow->setSurfaceType(QSurface::RasterSurface);
+    m_dummyWindow->show();
+    m_dummyWindowSurface = KWayland::Client::Surface::fromWindow(m_dummyWindow.data());
+}
+
+void WaylandServer::fakeDummyQtWindowInput()
+{
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0))
+    // we need to fake Qt into believing it has got any seat events
+    // this is done only when receiving either a key press or button.
+    // we simulate by sending a button press and release
+    auto surface = KWayland::Server::SurfaceInterface::get(m_dummyWindowSurface->id(), m_qtConnection);
+    if (!surface) {
+        return;
+    }
+    const auto oldSeatSurface = m_seat->focusedPointerSurface();
+    const auto oldPos = m_seat->focusedPointerSurfacePosition();
+    m_seat->setFocusedPointerSurface(surface, QPoint(0, 0));
+    m_seat->setPointerPos(QPointF(0, 0));
+    m_seat->pointerButtonPressed(Qt::LeftButton);
+    m_seat->pointerButtonReleased(Qt::LeftButton);
+    m_qtConnection->flush();
+    m_dummyWindow->hide();
+    m_seat->setFocusedPointerSurface(oldSeatSurface, oldPos);
+#endif
+}
+
 }

wayland_server.h

@@ -24,12 +24,15 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <QObject>
 
+class QWindow;
+
 namespace KWayland
 {
 namespace Client
 {
 class ConnectionThread;
 class ShmPool;
+class Surface;
 }
 namespace Server
 {
@@ -89,6 +92,7 @@ public:
      **/
     int createQtConnection();
     void createInternalConnection();
+    void createDummyQtWindow();
 
     KWayland::Server::ClientConnection *xWaylandConnection() const {
         return m_xwaylandConnection;
@@ -108,6 +112,7 @@ Q_SIGNALS:
     void shellClientRemoved(ShellClient*);
 
 private:
+    void fakeDummyQtWindowInput();
     KWayland::Server::Display *m_display = nullptr;
     KWayland::Server::CompositorInterface *m_compositor = nullptr;
     KWayland::Server::SeatInterface *m_seat = nullptr;
@@ -122,6 +127,8 @@ private:
     } m_internalConnection;
     AbstractBackend *m_backend = nullptr;
     QList<ShellClient*> m_clients;
+    QScopedPointer<QWindow> m_dummyWindow;
+    KWayland::Client::Surface *m_dummyWindowSurface = nullptr;
     KWIN_SINGLETON(WaylandServer)
 };