Throw if root node is collapsed

src/__tests__/__snapshots__/store-test.js.snap

@@ -264,3 +264,8 @@ exports[`Store collapseNodesByDefault:true should support mount and update opera
 `;
 
 exports[`Store collapseNodesByDefault:true should support mount and update operations: 3: unmount 1`] = ``;
+
+exports[`Store should not allow a root node to be collapsed: 1: mount 1`] = `
+[root]
+    <Component>
+`;

src/__tests__/store-test.js

@@ -22,7 +22,20 @@ describe('Store', () => {
   });
 
   it('should not allow a root node to be collapsed', () => {
-    // TODO
+    const Component = () => <div>Hi</div>;
+
+    act(() =>
+      ReactDOM.render(<Component count={4} />, document.createElement('div'))
+    );
+    expect(store).toMatchSnapshot('1: mount');
+
+    expect(store.roots).toHaveLength(1);
+
+    const rootID = store.roots[0];
+
+    expect(() => store.toggleIsCollapsed(rootID, true)).toThrow(
+      'Root nodes cannot be collapsed'
+    );
   });
 
   describe('collapseNodesByDefault:false', () => {

src/devtools/store.js

@@ -452,6 +452,10 @@ export default class Store extends EventEmitter {
   toggleIsCollapsed(id: number, isCollapsed: boolean): void {
     const element = this.getElementByID(id);
     if (element !== null) {
+      if (element.type === ElementTypeRoot) {
+        throw Error('Root nodes cannot be collapsed');
+      }
+
       const oldWeight = element.isCollapsed ? 1 : element.weight;
       element.isCollapsed = isCollapsed;
       const newWeight = element.isCollapsed ? 1 : element.weight;
@@ -576,7 +580,7 @@ export default class Store extends EventEmitter {
               depth: -1,
               displayName: null,
               id,
-              isCollapsed: false, // Never collapse roots
+              isCollapsed: false, // Never collapse roots; it would hide the entire tree.
               key: null,
               ownerID: 0,
               parentID: 0,