A self contained/instantiating Tooltip example for Unity3D, call with Tooltip.StartDisplay("some text")

Tooltip.cs

using UnityEngine;
using UnityEngine.UI;

[AddComponentMenu("")]
public class Tooltip : MonoBehaviour
{

    static Tooltip instance;

    TMPro.TextMeshProUGUI Text;
    Canvas canvas;
    RectTransform tooltipPanel;
    Vector3 offset = Vector3.up * 10;

    const float PADDING = 10;
    const float TEXT_PADDING = 5;
    const float SCREEN_BORDER = 5;

    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    static void Init()
    {
        GameObject instanceCanvas = new GameObject("STATIC_Tooltip");
        Canvas canvas = instanceCanvas.AddComponent<Canvas>();
        canvas.renderMode = RenderMode.ScreenSpaceOverlay;
        instance = instanceCanvas.AddComponent<Tooltip>();
        DontDestroyOnLoad(instanceCanvas);
        instance.canvas = canvas;
        //Canvas settings
        canvas.sortingOrder = 99;
        //Scaler
        CanvasScaler scaler = instanceCanvas.AddComponent<CanvasScaler>();
        scaler.uiScaleMode = CanvasScaler.ScaleMode.ScaleWithScreenSize;
        scaler.referenceResolution = new Vector2(3840, 2160);

        //Panel
        GameObject panel = new GameObject("Panel");
        panel.transform.SetParent(instanceCanvas.transform, false);
        Image panelImg = panel.AddComponent<Image>();
        panelImg.color = new Color(0.25f, 0.25f, 0.25f, 1f);
        //panel.StretchToParent(0, 1, 1, 1);
        VerticalLayoutGroup panelGroup = panel.AddComponent<VerticalLayoutGroup>();
        ContentSizeFitter panelSizeFitter = panel.AddComponent<ContentSizeFitter>();
        panelSizeFitter.verticalFit = ContentSizeFitter.FitMode.PreferredSize;
        panelSizeFitter.horizontalFit = ContentSizeFitter.FitMode.PreferredSize;
        panel.GetComponent<RectTransform>().pivot = new Vector2(0.5f, 0);
        instance.tooltipPanel = panel.GetComponent<RectTransform>();

        //BG Panel
        GameObject BGPanel = new GameObject("BGPanel");
        BGPanel.AddComponent<Image>().color = Color.black; //image also forces recttransform
        BGPanel.transform.SetParent(panel.transform, false);
        BGPanel.StretchToParent(0, 0, 1, 1);
        RectTransform bgRT = BGPanel.GetComponent<RectTransform>();
        bgRT.SetLeft(PADDING);
        bgRT.SetRight(PADDING);
        bgRT.SetBottom(PADDING);
        bgRT.SetTop(PADDING);
        LayoutElement BGLayoutElement = BGPanel.AddComponent<LayoutElement>();
        BGLayoutElement.ignoreLayout = true;

        //Text
        GameObject textOBJ = new GameObject("Text");
        instance.Text = textOBJ.AddComponent<TMPro.TextMeshProUGUI>(); //Forces rect transform
        textOBJ.transform.SetParent(panel.transform, false);
        textOBJ.StretchToParent(0, 0, 1, 1);
        instance.Text.color = Color.white;
        instance.Text.text = "<size=55>Potato</size><br><color=yellow>Rare</color><br><color=green>Use: Restores 5 health to user</color><br>Sell price: 9999";
        LayoutElement textLayoutElement = textOBJ.AddComponent<LayoutElement>();
        textLayoutElement.flexibleHeight = 1;
        ContentSizeFitter textSizeFitter = textOBJ.AddComponent<ContentSizeFitter>();
        textSizeFitter.verticalFit = ContentSizeFitter.FitMode.PreferredSize;
        instance.Text.margin = new Vector4(PADDING + TEXT_PADDING, PADDING + TEXT_PADDING, PADDING + TEXT_PADDING, PADDING + TEXT_PADDING);

        //Start hidden
        instance.gameObject.SetActive(false);
    }


    private void Update()
    {
        if (!gameObject.activeSelf)
            return;
        TrackCursor();
    }

    private void TrackCursor()
    {
        Vector3 newPos = Input.mousePosition + offset;
        newPos.z = 0f;
        float rightEdgeToScreenEdgeDistance = Screen.width - (newPos.x + tooltipPanel.rect.width * canvas.scaleFactor / 2) - SCREEN_BORDER;
        if (rightEdgeToScreenEdgeDistance < 0)
        {
            newPos.x += rightEdgeToScreenEdgeDistance;
        }
        float leftEdgeToScreenEdgeDistance = 0 - (newPos.x - tooltipPanel.rect.width * canvas.scaleFactor / 2) + SCREEN_BORDER;
        if (leftEdgeToScreenEdgeDistance > 0)
        {
            newPos.x += leftEdgeToScreenEdgeDistance;
        }
        float topEdgeToScreenEdgeDistance = Screen.height - (newPos.y + tooltipPanel.rect.height * canvas.scaleFactor) - SCREEN_BORDER;
        if (topEdgeToScreenEdgeDistance < 0)
        {
            newPos.y += topEdgeToScreenEdgeDistance;
        }
        tooltipPanel.transform.position = newPos;
    }


    public static void StartDisplay(string text)
    {
        instance.Text.text = text;
        instance.gameObject.SetActive(true);
        LayoutRebuilder.ForceRebuildLayoutImmediate(instance.tooltipPanel);
    }
    public static void StopDisplay()
    {
        instance.gameObject.SetActive(false);
    }
}