make a new component that only updates when the paint editor should update

src/containers/costume-tab.jsx

@@ -6,7 +6,7 @@ import VM from 'scratch-vm';
 
 import AssetPanel from '../components/asset-panel/asset-panel.jsx';
 import addCostumeIcon from '../components/asset-panel/icon--add-costume-lib.svg';
-import PaintEditor from 'scratch-paint';
+import PaintEditorWrapper from './paint-editor-wrapper.jsx';
 
 import {connect} from 'react-redux';
 
@@ -20,8 +20,7 @@ class CostumeTab extends React.Component {
         super(props);
         bindAll(this, [
             'handleSelectCostume',
-            'handleDeleteCostume',
-            'handleUpdateSvg'
+            'handleDeleteCostume'
         ]);
         this.state = {selectedCostumeIndex: 0};
     }
@@ -39,25 +38,6 @@ class CostumeTab extends React.Component {
         }
     }
 
-    shouldComponentUpdate (nextProps) {
-        const {
-            editingTarget,
-            sprites,
-            stage
-        } = nextProps;
-        const nextTarget = editingTarget && sprites[editingTarget] ? sprites[editingTarget] : stage;
-        const currentTarget =
-            this.props.editingTarget && this.props.sprites[this.props.editingTarget] ?
-                this.props.sprites[this.props.editingTarget] :
-                this.props.stage;
-
-        if (this.props.editingTarget !== editingTarget ||
-                currentTarget.currentCostume !== nextTarget.currentCostume) {
-            return true;
-        }
-        return false;
-    }
-
     handleSelectCostume (costumeIndex) {
         this.props.vm.editingTarget.setCostume(costumeIndex);
         this.setState({selectedCostumeIndex: costumeIndex});
@@ -67,19 +47,20 @@ class CostumeTab extends React.Component {
         this.props.vm.deleteCostume(costumeIndex);
     }
 
-    handleUpdateSvg (svg, rotationCenterX, rotationCenterY) {
-        this.props.vm.updateSvg(this.state.selectedCostumeIndex, svg, rotationCenterX, rotationCenterY);
-    }
-
     render () {
+        // For paint wrapper
         const {
-            editingTarget,
-            sprites,
-            stage,
+            onNewBackdropClick,
             onNewCostumeClick,
-            onNewBackdropClick
+            ...props
         } = this.props;
 
+        const {
+            editingTarget,
+            sprites,
+            stage
+        } = props;
+
         const target = editingTarget && sprites[editingTarget] ? sprites[editingTarget] : stage;
 
         if (!target) {
@@ -117,11 +98,9 @@ class CostumeTab extends React.Component {
                 onItemClick={this.handleSelectCostume}
             >
                 {target.costumes ?
-                    <PaintEditor
-                        rotationCenterX={target.costumes[this.state.selectedCostumeIndex].rotationCenterX}
-                        rotationCenterY={target.costumes[this.state.selectedCostumeIndex].rotationCenterY}
-                        svg={this.props.vm.getCostumeSvg(this.state.selectedCostumeIndex)}
-                        onUpdateSvg={this.handleUpdateSvg}
+                    <PaintEditorWrapper
+                        {...props}
+                        selectedCostumeIndex={this.state.selectedCostumeIndex}
                     /> :
                     null
                 }

src/containers/paint-editor-wrapper.jsx

+import PropTypes from 'prop-types';
+import React from 'react';
+import bindAll from 'lodash.bindall';
+import VM from 'scratch-vm';
+
+import PaintEditor from 'scratch-paint';
+
+import {connect} from 'react-redux';
+
+class PaintEditorWrapper extends React.Component {
+    constructor (props) {
+        super(props);
+        bindAll(this, [
+            'handleUpdateSvg'
+        ]);
+    }
+    shouldComponentUpdate (nextProps) {
+        // Only update on sprite change or costume change. No need to push the SVG to the paint
+        // editor that it just exported; it causes the paint editor to lose some state.
+        const {
+            editingTarget,
+            sprites,
+            stage
+        } = nextProps;
+        const nextTarget = editingTarget && sprites[editingTarget] ? sprites[editingTarget] : stage;
+        const currentTarget =
+            this.props.editingTarget && this.props.sprites[this.props.editingTarget] ?
+                this.props.sprites[this.props.editingTarget] :
+                this.props.stage;
+
+        if (this.props.editingTarget !== editingTarget ||
+                currentTarget.currentCostume !== nextTarget.currentCostume) {
+            return true;
+        }
+        return false;
+    }
+    handleUpdateSvg (svg, rotationCenterX, rotationCenterY) {
+        this.props.vm.updateSvg(this.props.selectedCostumeIndex, svg, rotationCenterX, rotationCenterY);
+    }
+    render () {
+        const {
+            editingTarget,
+            sprites,
+            stage
+        } = this.props;
+
+        const target = editingTarget && sprites[editingTarget] ? sprites[editingTarget] : stage;
+
+        if (!target || !target.costumes) {
+            return null;
+        }
+
+        return (
+            <PaintEditor
+                rotationCenterX={target.costumes[this.props.selectedCostumeIndex].rotationCenterX}
+                rotationCenterY={target.costumes[this.props.selectedCostumeIndex].rotationCenterY}
+                svg={this.props.vm.getCostumeSvg(this.props.selectedCostumeIndex)}
+                onUpdateSvg={this.handleUpdateSvg}
+            />
+        );
+    }
+}
+
+PaintEditorWrapper.propTypes = {
+    editingTarget: PropTypes.string,
+    selectedCostumeIndex: PropTypes.number.isRequired,
+    sprites: PropTypes.shape({
+        id: PropTypes.shape({
+            costumes: PropTypes.arrayOf(PropTypes.shape({
+                url: PropTypes.string,
+                name: PropTypes.string.isRequired
+            }))
+        })
+    }),
+    stage: PropTypes.shape({
+        sounds: PropTypes.arrayOf(PropTypes.shape({
+            name: PropTypes.string.isRequired
+        }))
+    }),
+    vm: PropTypes.instanceOf(VM)
+};
+
+const mapStateToProps = state => ({
+    editingTarget: state.targets.editingTarget,
+    sprites: state.targets.sprites,
+    stage: state.targets.stage,
+    costumeLibraryVisible: state.modals.costumeLibrary,
+    backdropLibraryVisible: state.modals.backdropLibrary
+});
+
+export default connect(
+    mapStateToProps
+)(PaintEditorWrapper);