Is the WindowProcedure always called in the JavaFX Application Thread?

In a JavaFX app, if I use the JNA to implement a custom window procedure, will the callback always be on the JavaFX Application Thread?

public class MyWindowProc implements WinUser.WindowProc

...

    @Override
    public WinDef.LRESULT callback(WinDef.HWND hwnd, int uMsg, WinDef.WPARAM wParam, WinDef.LPARAM lParam) {
        // Is this guaranteed to always be true?
        System.out.println(Platform.isFxApplicationThread());

    ...
}

I assumed it wouldn't, but when I call Platform.isFxApplicationThread() I always get true. Is this guaranteed to always be the case?