Editor script to toggle on/off the "UI" layer when switching Unity Editor's scene view perspective (2D to 3D)... To avoid the huge UI flying over your 3D work

SceneViewUILayerToggle.cs

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class SceneViewUILayerToggle
{
    // Define the UI layer's mask
    private static readonly int uiLayerMask = 1 << LayerMask.NameToLayer("UI");

    // Constructor to initialize the script
    static SceneViewUILayerToggle()
    {
        // Subscribe to the SceneView.duringSceneGui event, which gets called every frame in the Scene view
        SceneView.duringSceneGui += OnSceneGUI;
    }

    private static bool wasInitialized = false;
    private static bool wasIn2DMode = true;
    private static void OnSceneGUI(SceneView sceneView)
    {
        if (wasIn2DMode != sceneView.in2DMode || wasInitialized == false)
        {
            // Reset flags
            wasInitialized = true;
            wasIn2DMode = sceneView.in2DMode;

            // Update view
            {
                // Check if the scene view is in 2D mode
                if (sceneView.in2DMode)
                {
                    // If in 2D mode, make sure the UI layer is visible
                    SetLayerVisibility(sceneView, true);
                }
                else
                {
                    // If not in 2D mode (i.e., 3D mode), make sure the UI layer is hidden
                    SetLayerVisibility(sceneView, false);
                }
            }
        }
    }

    private static void SetLayerVisibility(SceneView sceneView, bool showUILayer)
    {
        // Get the current scene view layer mask
        int currentMask = Tools.visibleLayers;

        // If we want to show the UI layer
        if (showUILayer)
        {
            // Add the UI layer to the mask if it's not already visible
            Tools.visibleLayers = currentMask | uiLayerMask;
        }
        else
        {
            // Remove the UI layer from the mask if it's visible
            Tools.visibleLayers = currentMask & ~uiLayerMask;
        }
    }
}