using Sandbox;
using Sandbox.UI;

namespace NZombies;

/// <summary>
/// A title bar you can drag a dev panel around by. `&lt;DragHandle class="head"&gt;`.
///
/// ⛔ A PANEL SUBCLASS RATHER THAN RAZOR EVENT HANDLERS, AND THE REASON IS MOUSE CAPTURE. A
/// `onmousedown` in the markup gives you the press but nothing that survives the cursor leaving
/// the bar — and a window drag is precisely the gesture where the cursor outruns the thing it is
/// dragging. `Panel.HasActive` stays true from press to release no matter where the pointer goes,
/// which is what makes the drag feel like a window instead of like a slider that keeps slipping.
///
/// ⚠️ IT MOVES THE NEAREST ANCESTOR WITH CLASS `panel`, not its own parent. The grip sits
/// inside the title bar, which sits inside the window, so "my parent" is the bar and dragging it
/// would slide the bar around inside a window that never moved. Walking up to a named class
/// survives someone adding another wrapper later, which a hard-coded `Parent.Parent` would not.
///
/// ⚠️ SCREEN PIXELS AND PANEL UNITS ARE NOT THE SAME NUMBER. `Box.Rect` and `Mouse.Position` are
/// real pixels; `Style.Left` is in the panel's own scaled units. `ScaleFromScreen` is the
/// conversion, the same one SniperScope and DamageNumbersHud already use — without it the window
/// runs away from the cursor on any display that is not at 100%.
/// </summary>
public class DragHandle : Panel
{
	/// <summary>What to move. Null walks up to the nearest ancestor with class `panel`.</summary>
	public Panel Target { get; set; }

	Panel Moving
	{
		get
		{
			if ( Target is not null ) return Target;

			for ( var p = Parent; p is not null; p = p.Parent )
				if ( p.HasClass( "panel" ) ) return p;

			return Parent;
		}
	}

	// The cursor's offset from the moved panel's top-left, in screen pixels, captured on press.
	// ⚠️ WITHOUT THIS THE WINDOW SNAPS ITS CORNER TO THE POINTER on the first frame of every drag,
	// which reads as the panel jumping out from under you before it starts following.
	Vector2 _grab;

	protected override void OnMouseDown( MousePanelEvent e )
	{
		base.OnMouseDown( e );

		// ⚠️ THE CLOSE BUTTON IS NOT COVERED BY THE GRIP, which is handled in the stylesheet by
		// stopping this element short of the bar's right edge rather than by testing the target
		// here. A geometric answer cannot be defeated by an event arriving from an unexpected
		// panel, and it also keeps the button's own hover state working.
		var t = Moving;
		if ( t is null ) return;

		_grab = Mouse.Position - new Vector2( t.Box.Rect.Left, t.Box.Rect.Top );
		e.StopPropagation();
	}

	protected override void OnMouseMove( MousePanelEvent e )
	{
		base.OnMouseMove( e );

		if ( !HasActive ) return;

		var t = Moving;
		if ( t is null ) return;

		var s = ScaleFromScreen;
		var pos = (Mouse.Position - _grab) * s;

		// ⛔ CLAMPED SO THE BAR CANNOT LEAVE THE SCREEN. A panel dragged off the top or past an
		// edge takes its own title bar with it, and a window you cannot grab is a window you
		// cannot get back without a console command. The limits keep the bar reachable rather
		// than keeping the whole panel visible, which would stop you tucking one into a corner.
		var w = Screen.Width * s;
		var h = Screen.Height * s;
		var keep = 80f;

		pos.x = pos.x.Clamp( keep - t.Box.Rect.Width * s, w - keep );
		pos.y = pos.y.Clamp( 0f, h - keep );

		t.Style.Left = Length.Pixels( pos.x );
		t.Style.Top = Length.Pixels( pos.y );

		e.StopPropagation();
	}
}