particles/MainMenu/MainMenu.cs

using Godot;
using System;

public class MainMenu : Control
{

    // Nodes for menu items
    private Main _main;
    private Button _simulateButton;
    private Button _toggleFullScreenButton;
    private TextEdit _seedText;
    private TextEdit _particleCountText;

    public override void _Ready()
    {
        // Init nodes
        _main = GetTree().Root.GetNode<Main>("Main");
        _simulateButton = GetNode("MenuButtons").GetNode<Button>("SimulateButton");
        _toggleFullScreenButton = GetNode("MenuButtons").GetNode<Button>("ToggleFullscreenButton");
        _seedText = GetNode("MenuButtons").GetNode<TextEdit>("SeedText");
        _particleCountText = GetNode("MenuButtons").GetNode<TextEdit>("ParticleCountText");
        
        // Connect signals
        _simulateButton.Connect("pressed", this, nameof(_OnSimulatePressed));
        _toggleFullScreenButton.Connect("pressed", this, nameof(_OnToggleFullscreenPressed));
        
        // Random seed
        GD.Randomize();
        var randomSeed = (int)GD.Randi();
       _seedText.Text = randomSeed.ToString();
    }

    public void _OnSimulatePressed()
    {
        // Start simulation with seed and number of particles
        var nParticles = _particleCountText.Text.ToInt();
        var seed = _seedText.Text.ToInt();
        _main.StartSimulation(seed, nParticles);
    }

    public void _OnToggleFullscreenPressed()
    {
        OS.WindowFullscreen = !OS.WindowFullscreen;
    }

    public override void _Input(InputEvent @event)
    {
        // Quit only if menu is visible
        if (Visible && @event.IsActionPressed("quit"))
        {
            GetTree().Quit();
        }
    }
}