Iphone X margins?

MacOSX and iOS
3

you just have to hard-code it:

#if JUCE_IOS
    DBG( "device description: " << SystemStats::getDeviceDescription() );
    auto platform = getPlatform();
    if( platform == "iPhone10,3" || platform == "iPhone10,6")
    {
        //it's an iPhone X
        setContentOwned(mcc = new iPhoneXContentComponent(), true);
        setUsingNativeTitleBar(true);
    }
    else
    {
        setContentOwned(mcc = new MainContentComponent(), true);
        setUsingNativeTitleBar(true);
    }
#else
    setContentOwned(mcc = new MainContentComponent(), true);
    setUsingNativeTitleBar(true);
#endif

and

struct iPhoneXContentComponent : public Component
{
    ScopedPointer<ChordieApp::MainContentComponent> mcc;
    iPhoneXContentComponent();
    void paint(Graphics& g) override;
    void resized() override;
};

iPhoneXContentComponent::iPhoneXContentComponent()
{
    addAndMakeVisible( mcc = new ChordieApp::MainContentComponent() );
    auto area = Desktop::getInstance().getDisplays().getMainDisplay().totalArea;
    setSize(area.getWidth(), area.getHeight());
    DBG( SystemStats::getDeviceDescription() );
}

void iPhoneXContentComponent::paint(Graphics& g)
{
    g.fillAll(Colours::white);
}

void iPhoneXContentComponent::resized()
{
    //I figured out these values visually by hand via the Simulator for Landscape mode
    mcc->setBounds(40, 0, getWidth() - 80, getHeight() - 20);
}

see Get iOS Device for the getPlatform() code