Fix preview rendering bugs (causing re-use of previous texture and disapperance of checkerboard background) and persistence (remembers zoom etc. between assembly reloads, scene changes, even play mode!)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphEditWindow.cs

                     m_GraphEditorView.onUpdateAssetClick += UpdateAsset;
                     m_GraphEditorView.onConvertToSubgraphClick += ToSubGraph;
                     m_GraphEditorView.onShowInProjectClick += PingAsset;
-                    m_GraphEditorView.RegisterCallback<PostLayoutEvent>(OnPostLayout);
                     rootVisualContainer.Add(graphEditorView);
                 }
             }
             if (materialGraph == null)
                 return;
             if (graphEditorView == null)
-                graphEditorView = new GraphEditorView(materialGraph, selected);
+                graphEditorView = new GraphEditorView(materialGraph, selected) { persistenceKey = AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(selected)) };
             if (graphEditorView != null)
                 graphEditorView.previewSystem.Update();
         }
             graphObject.graph.OnEnable();
             graphObject.graph.ValidateGraph();
 
-            graphEditorView = new GraphEditorView(m_GraphObject.graph as AbstractMaterialGraph, selected);
+            graphEditorView = new GraphEditorView(m_GraphObject.graph as AbstractMaterialGraph, selected) { persistenceKey = AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(selected)) };
+            graphEditorView.RegisterCallback<PostLayoutEvent>(OnPostLayout);
             titleContent = new GUIContent(selected.name);
 
             Repaint();

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphPreviewGenerator.cs

             Light1.color = new Color(.4f, .4f, .45f, 0f) * .7f;
 
             m_CheckerboardMaterial = new Material(Shader.Find("Hidden/Checkerboard"));
-            m_CheckerboardMaterial.SetFloat("_X", 32);
-            m_CheckerboardMaterial.SetFloat("_Y", 32);
+            m_CheckerboardMaterial.shader.hideFlags = HideFlags.HideAndDontSave;
+            m_CheckerboardMaterial.hideFlags = HideFlags.HideAndDontSave;
 
             if (s_Meshes[0] == null)
             {
             RenderSettings.ambientProbe = ambientProbe;
 
             m_Camera.targetTexture = renderTexture;
+            var previousRenderTexure = RenderTexture.active;
             RenderTexture.active = renderTexture;
             GL.Clear(true, true, Color.black);
             Graphics.Blit(Texture2D.whiteTexture, renderTexture, m_CheckerboardMaterial);
             m_Camera.Render();
             Unsupported.useScriptableRenderPipeline = oldAllowPipes;
 
+            RenderTexture.active = previousRenderTexure;
             Unsupported.RestoreOverrideRenderSettings();
 
             Light0.enabled = false;
         public void Dispose()
         {
-            if (Light0 == null)
+            if (Light0 != null)
-            if (Light1 == null)
+            if (Light1 != null)
-            if (m_Camera == null)
+            if (m_Camera != null)
-            if (m_CheckerboardMaterial == null)
+            if (m_CheckerboardMaterial != null)
             {
                 UnityEngine.Object.DestroyImmediate(m_CheckerboardMaterial);
                 m_CheckerboardMaterial = null;

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/PreviewSystem.cs

         {
             m_Graph = graph;
             m_PreviewMaterial = new Material(Shader.Find("Unlit/Color")) { hideFlags = HideFlags.HideInHierarchy };
-            m_PreviewMaterial.hideFlags = HideFlags.HideInHierarchy;
+            m_PreviewMaterial.hideFlags = HideFlags.HideAndDontSave;
             m_PreviewPropertyBlock = new MaterialPropertyBlock();
             m_ErrorTexture = new Texture2D(2, 2);
             m_ErrorTexture.SetPixel(0, 0, Color.magenta);

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/GraphEditorView.cs

 
             var content = new VisualElement { name = "content" };
             {
-                m_GraphView = new MaterialGraphView(graph) { name = "GraphView" };
-                m_GraphView.persistenceKey = AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(asset));
+                m_GraphView = new MaterialGraphView(graph) { name = "GraphView", persistenceKey = "MaterialGraphView" };
                 m_GraphView.SetupZoom(ContentZoomer.DefaultMinScale, ContentZoomer.DefaultMaxScale);
                 m_GraphView.AddManipulator(new ContentDragger());
                 m_GraphView.AddManipulator(new RectangleSelector());
             onUpdateAssetClick = null;
             onConvertToSubgraphClick = null;
             onShowInProjectClick = null;
+            m_Graph.onChange -= OnGraphChange;
             if (m_GraphInspectorView != null) m_GraphInspectorView.Dispose();
             if (previewSystem != null)
             {

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Resources/Shaders/Checkerboard.shader

 
             #include "UnityCG.cginc"
 
-            uniform float _X;
-            uniform float _Y;
+            static const float rows = 32;
+            static const float columns = 32;
 
             static const float4 col1 = float4(0.15, 0.15, 0.15, 1.0);
             static const float4 col2 = float4(0.3, 0.3, 0.3, 1.0);
-                float total = floor(i.uv.x * _X) + floor(i.uv.y * _Y);
+                float total = floor(i.uv.x * rows) + floor(i.uv.y * columns);
                 bool isEven = total % 2.0 == 0.0;
                 return isEven ? col1 : col2;
             }