Example showing how to create a collapsible panel in Eto using C#

EtoCollapsibleDialog.cs

using System;
using Eto.Forms;
using Eto.Drawing;

namespace MyCollapserForm
{
	public class MainForm : Dialog<bool>
	{
		class L : Label
		{
			public L(string text)
			{
				VerticalAlignment = VerticalAlignment.Center;
				TextAlignment = TextAlignment.Right;
				Text = text;
			}
		}

		public MainForm()
		{
			Title = "My Collapsible Eto Form";
			Resizable = false;
			Padding = new Padding(5);

			var collapsePanel = new DynamicLayout { Visible = false, Padding = new Padding(40, 10), DefaultSpacing = new Size(5,5) };

			// set content of the collapsed section
			collapsePanel.BeginVertical();
			collapsePanel.AddRow(null, new L("Density:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Maximum angle:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Maximum aspect ratio:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Minimum edge length:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Maximum edge length:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Maximum distance, edge to surface:"), new NumericUpDown());
			collapsePanel.AddRow(null, new L("Minimum initial grid quads:"), new NumericUpDown());
			collapsePanel.EndVertical();

			collapsePanel.BeginVertical();
			collapsePanel.AddRow(null, new CheckBox { Text = "Refine mesh" });
			collapsePanel.AddRow(null, new CheckBox { Text = "Jagged seams" }, new CheckBox { Text = "Pack textures" }, null);
			collapsePanel.AddRow(null, new CheckBox { Text = "Simple planes" });
			collapsePanel.EndVertical();

			// button to toggle collapsing
			var collapseButton = new Button { Text = "v", MinimumSize = Size.Empty };
			collapseButton.Click += (sender, e) =>
			{
				collapsePanel.Visible = !collapsePanel.Visible;
				collapseButton.Text = collapsePanel.Visible ? "^" : "v";

				// adjust form size
				if (collapsePanel.Visible)
					ClientSize = new Size(Math.Max(ClientSize.Width, collapsePanel.Width), ClientSize.Height + collapsePanel.Height);
				else
					ClientSize = new Size(ClientSize.Width, ClientSize.Height - collapsePanel.Height);
			};

			// a few buttons always shown at the bottom
			var previewButton = new Button { Text = "Preview" };

			var cancelButton = new Button { Text = "Cancel" };
			cancelButton.Click += (sender, e) => Close(false);

			var okButton = new Button { Text = "OK" };
			okButton.Click += (sender, e) => Close(true);

			DefaultButton = okButton;
			AbortButton = cancelButton;

			var toleranceUpDown = new NumericUpDown();

			// our main layout
			var layout = new DynamicLayout();
			layout.AddSeparateRow(null, new L("Tolerance"), toleranceUpDown, new L("millimeters"), collapseButton);
			layout.AddCentered(collapsePanel); // we need this auto-sized so we can get its width to adjust form height
			layout.Add(null); // expanding space, in case you want the form re-sizable
			layout.AddSeparateRow(null, previewButton, cancelButton, okButton);
			Content = layout;
		}
	}
}