shmodot/scripts/OrbitCamera.cs

using Godot;
using System;

public class OrbitCamera : Spatial
{
    [Export]
    private float _sensitivity = 200f;

    private Vector2 _rotation;
    private bool _canRotate = false;

    public override void _Input(InputEvent e)
    {
        if (e is InputEventMouseMotion mouseMotion)
        {
            HandleMouseMovement(mouseMotion);
        }
        if (e is InputEventMouseButton mouseButton)
        {
            HandleMouseButton(mouseButton);
        }
    }

    public override void _Process(float delta)
    {
        Rotation = Vector3.Zero;
        var sensitivity = -1f / _sensitivity;
        Rotate(Vector3.Right, _rotation.y * sensitivity);
        Rotate(Vector3.Up, _rotation.x * sensitivity);
    }

    // left click to drag
    private void HandleMouseButton(InputEventMouseButton mouseButton)
    {
        if (mouseButton.ButtonIndex != (int)ButtonList.Left) return;

        _canRotate = mouseButton.Pressed;
    }

    private void HandleMouseMovement(InputEventMouseMotion mouseMotion)
    {
        if (!_canRotate) return;

        var delta = mouseMotion.Relative;
        _rotation += delta;
    }
}