Qt5 QDockWidget with Fixed Height CentralWidget

I'm trying to use dock widgets on the main window class in Qt5. However, when I set the central widget to have a fixed height Qt has trouble docking the windows to the top or bottom. Basically, it looks like there is some "padding" or "margins" above and below the central widget. If I set an auto height on the widget, the docking works fine all they way edge-to-edge (top/bottom). How can I either remove the margins or enable the docking function while using a fixed height central widget?

See screenshots for example.

Dock Right w/ Auto Height (No Margins on Central Widget)

Dock Bottom w/ Auto Height (No Margins on Central Widget)

Dock Bottom w/ Fixed Height (Margins/Padding--Grey areas, won't dock)

Here is the code if that helps.

Header:

class MainWindow : public QMainWindow {
    Q_OBJECT

public:
    MainWindow(QString);
    ~MainWindow();

private:
    void createDockWindows();
    QListWidget *m_dock_list;
    QString m_directory;
    QWidget *m_mainWidget;
};

Class definition:

MainWindow::MainWindow(QString program)
    : m_directory(".")
{
    m_mainWidget = new QWidget;
    m_mainWidget->setFixedHeight(156);
    m_mainWidget->setStyleSheet("background-color: blue;");

    createDockWindows();

    // set central widget and default size
    setCentralWidget(m_mainWidget);
}

// dock functions
void MainWindow::createDockWindows()
{
    QDockWidget *dock = new QDockWidget(tr("Dock List"), this);
    dock->setAllowedAreas(Qt::LeftDockWidgetArea | 
                          Qt::RightDockWidgetArea |
                          Qt::BottomDockWidgetArea);
    m_dock_list = new QListWidget(dock);
    m_dock_list->addItems(QStringList()
        << "item 1"
        << "item 2"
        << "item 3"
        << "item 4");
    dock->setWidget(m_dock_list);
    addDockWidget(Qt::RightDockWidgetArea, dock);
}