ResizeHandle.cs

using Godot;
using System;

public enum ResizeHandleType
{
	Top,
	Bottom,
	Left,
	Right
}

public class ResizeHandle : Control
{
	[Export]
	private ResizeHandleType handleType;
	private bool isResizing = false;
	private Vector2 resizePos = Vector2.Zero;
	private Vector2 startSize = Vector2.Zero;

	public override void _Ready()
	{
		switch(handleType)
		{
			case ResizeHandleType.Top:
			case ResizeHandleType.Bottom:
				MouseDefaultCursorShape = CursorShape.Vsize;
				break;
			case ResizeHandleType.Left:
			case ResizeHandleType.Right:
				MouseDefaultCursorShape = CursorShape.Hsize;
				break;
		}
	}

	public override void _GuiInput(InputEvent ev)
	{
		if(ev is InputEventMouseButton button)
		{
			if(button.ButtonIndex == (int)ButtonList.Left)
			{
				if(button.Pressed && !isResizing) {
					isResizing = true;
					startSize = OS.WindowSize;
					resizePos = GetGlobalMousePosition();
				}
				else
					isResizing = false;
			}
		}
	}

	public override void _Process(float delta)
	{
		if(isResizing)
		{
			var mousePos = GetGlobalMousePosition();
			switch(handleType)
			{
				case ResizeHandleType.Top:
				{
					var newPos = OS.WindowPosition + mousePos - resizePos;
					newPos.x = OS.WindowPosition.x;
					var deltaPos = OS.WindowPosition - newPos;
					startSize += new Vector2(0.0f, deltaPos.y);
					OS.WindowPosition = newPos;
					OS.WindowSize = startSize;
					break;
				}
				case ResizeHandleType.Bottom:
					OS.WindowSize = startSize + new Vector2(0.0f, mousePos.y - resizePos.y);
					break;
				case ResizeHandleType.Left:
				{
					var newPos = OS.WindowPosition + mousePos - resizePos;
					newPos.y = OS.WindowPosition.y;
					var deltaPos = OS.WindowPosition - newPos;
					startSize += new Vector2(deltaPos.x, 0.0f);
					OS.WindowPosition = newPos;
					OS.WindowSize = startSize;
					break;
				}
				case ResizeHandleType.Right:
					OS.WindowSize = startSize + new Vector2(mousePos.x - resizePos.x, 0.0f);
					break;
			}
		}
	}
}