Quickly toggle the visibility of UI layer (canvases) in Unity's Scene view

UIToggler.cs

using UnityEditor;
using UnityEngine;

public class UIToggler
{
	private const int UI_LAYER = 1 << 5;

	[InitializeOnLoadMethod]
	private static void Init()
	{
#if UNITY_2019_1_OR_NEWER
		SceneView.duringSceneGui -= OnSceneGUI;
		SceneView.duringSceneGui += OnSceneGUI;
#else
		SceneView.onSceneGUIDelegate -= OnSceneGUI;
		SceneView.onSceneGUIDelegate += OnSceneGUI;
#endif
	}

	private static void OnSceneGUI( SceneView sceneView )
	{
		Handles.BeginGUI();

		bool uiVisible = ( Tools.visibleLayers & UI_LAYER ) == UI_LAYER;
		if( GUI.Button( new Rect( 0f, 0f, 125f, 25f ), uiVisible ? "Hide Canvas" : "Show Canvas" ) )
		{
			if( uiVisible )
			{
				Tools.visibleLayers &= ~UI_LAYER;
				Tools.lockedLayers |= UI_LAYER;
			}
			else
			{
				Tools.visibleLayers |= UI_LAYER;
				Tools.lockedLayers &= ~UI_LAYER;
			}
		}

		Handles.EndGUI();
	}
}

How To

Simply create a folder called Editor inside your Project window and add this script inside it. Then, click the small button at the top left corner of the Scene view to toggle the UI objects' visibility in Scene view.