Sandbox.UI.Panel

// Create panel dynamically
var panel = new Panel();
panel.AddClass("my-panel");

// Add to root
RootPanel.Current.AddChild(panel);

// Add child
var child = panel.AddChild<Button>();

// Remove child
panel.DeleteChildren();

// Find child
var button = panel.QuerySelector(".submit-btn");

// Add CSS classes
panel.AddClass("visible");

// Remove class
panel.RemoveClass("hidden");

// Toggle
panel.ToggleClass("active");

// Set styles directly
panel.Style.BackgroundColor = Color.Red;
panel.Style.Width = Length.Percent(50);

// Flex layout (default)
panel.Style.FlexDirection = FlexDirection.Column;
panel.Style.JustifyContent = Justify.Center;
panel.Style.AlignItems = Align.Center;

// Absolute positioning
panel.Style.Position = PositionMode.Absolute;
panel.Style.Left = 100;
panel.Style.Top = 50;

// Mouse events
panel.AddEventListener("onmousedown", (e) => {
    Log.Info("Mouse down!");
});

// Keyboard
panel.AddEventListener("onkeydown", (e) => {
    if (e.Key == "escape") Close();
});

// Show/hide
panel.SetClass("hidden", false); // Show
panel.SetClass("hidden", true);  // Hide

// Check visibility
bool visible = panel.IsVisible;

protected override void OnAfterTreeRender(bool firstTime)
{
    if (firstTime)
    {
        // Tree is ready
    }
}