Check for frozen style object before List modifies row-style

Resolves bvaughn#556

Author: Brian Vaughn

source/List/List.js

@@ -158,9 +158,17 @@ export default class List extends Component {
   _cellRenderer ({ rowIndex, style, ...rest }) {
     const { rowRenderer } = this.props
 
-    // By default, List cells should be 100% width.
-    // This prevents them from flowing under a scrollbar (if present).
-    style.width = '100%'
+    // TRICKY The style object is sometimes cached by Grid.
+    // This prevents new style objects from bypassing shallowCompare().
+    // However as of React 16, style props are auto-frozen (at least in dev mode)
+    // Check to make sure we can still modify the style before proceeding.
+    // https://github.com/facebook/react/commit/977357765b44af8ff0cfea327866861073095c12#commitcomment-20648713
+    const { writable } = Object.getOwnPropertyDescriptor(style, 'width')
+    if (writable) {
+      // By default, List cells should be 100% width.
+      // This prevents them from flowing under a scrollbar (if present).
+      style.width = '100%'
+    }
 
     return rowRenderer({
       index: rowIndex,