Fix tab panel initial tab selection (#47100)

* Fix tab panel initial tab selection

* Add changelog entry

* Add a test case for a deferred initial tab

packages/components/CHANGELOG.md

@@ -15,6 +15,7 @@
 
 -   `Placeholder`: set fixed right margin for label's icon ([46918](https://github.com/WordPress/gutenberg/pull/46918)).
 -   `TreeGrid`: Fix right-arrow keyboard navigation when a row contains more than two focusable elements ([46998](https://github.com/WordPress/gutenberg/pull/46998)).
+-   `TabPanel`: Fix initial tab selection when the tab declaration is lazily added to the `tabs` array ([47100](https://github.com/WordPress/gutenberg/pull/47100)).
 
 ## 23.1.0 (2023-01-02)
 

packages/components/src/tab-panel/index.tsx

@@ -103,25 +103,47 @@ export function TabPanel( {
 	const selectedTab = tabs.find( ( { name } ) => name === selected );
 	const selectedId = `${ instanceId }-${ selectedTab?.name ?? 'none' }`;
 
+	// Handle selecting the initial tab.
 	useEffect( () => {
-		const firstEnabledTab = tabs.find( ( tab ) => ! tab.disabled );
+		// If there's a selected tab, don't override it.
+		if ( selectedTab ) {
+			return;
+		}
+
 		const initialTab = tabs.find( ( tab ) => tab.name === initialTabName );
-		if ( ! selectedTab?.name && firstEnabledTab ) {
-			handleTabSelection(
-				initialTab && ! initialTab.disabled
-					? initialTab.name
-					: firstEnabledTab.name
-			);
-		} else if ( selectedTab?.disabled && firstEnabledTab ) {
+
+		// Wait for the denoted initial tab to be declared before making a
+		// selection. This ensures that if a tab is declared lazily it can
+		// still receive initial selection.
+		if ( initialTabName && ! initialTab ) {
+			return;
+		}
+
+		if ( initialTab && ! initialTab.disabled ) {
+			// Select the initial tab if it's not disabled.
+			handleTabSelection( initialTab.name );
+		} else {
+			// Fallback to the first enabled tab when the initial is disabled.
+			const firstEnabledTab = tabs.find( ( tab ) => ! tab.disabled );
+			if ( firstEnabledTab ) handleTabSelection( firstEnabledTab.name );
+		}
+	}, [ tabs, selectedTab, initialTabName, handleTabSelection ] );
+
+	// Handle the currently selected tab becoming disabled.
+	useEffect( () => {
+		// This effect only runs when the selected tab is defined and becomes disabled.
+		if ( ! selectedTab?.disabled ) {
+			return;
+		}
+
+		const firstEnabledTab = tabs.find( ( tab ) => ! tab.disabled );
+
+		// If the currently selected tab becomes disabled, select the first enabled tab.
+		// (if there is one).
+		if ( firstEnabledTab ) {
 			handleTabSelection( firstEnabledTab.name );
 		}
-	}, [
-		tabs,
-		selectedTab?.name,
-		selectedTab?.disabled,
-		initialTabName,
-		handleTabSelection,
-	] );
+	}, [ tabs, selectedTab?.disabled, handleTabSelection ] );
 
 	return (
 		<div className={ className }>

packages/components/src/tab-panel/test/index.tsx

@@ -147,7 +147,7 @@ describe( 'TabPanel', () => {
 		);
 	} );
 
-	it( 'should select `initialTabname` if defined', () => {
+	it( 'should select `initialTabName` if defined', () => {
 		const mockOnSelect = jest.fn();
 
 		render(
@@ -162,6 +162,39 @@ describe( 'TabPanel', () => {
 		expect( mockOnSelect ).toHaveBeenLastCalledWith( 'beta' );
 	} );
 
+	it( 'waits for the tab with the `initialTabName` to become present in the `tabs` array before selecting it', () => {
+		const mockOnSelect = jest.fn();
+
+		const { rerender } = render(
+			<TabPanel
+				tabs={ TABS }
+				initialTabName="delta"
+				children={ () => undefined }
+				onSelect={ mockOnSelect }
+			/>
+		);
+
+		// There should be no selected tab.
+		expect(
+			screen.queryByRole( 'tab', { selected: true } )
+		).not.toBeInTheDocument();
+
+		rerender(
+			<TabPanel
+				tabs={ [
+					{ name: 'delta', title: 'Delta', className: 'delta-class' },
+					...TABS,
+				] }
+				initialTabName="delta"
+				children={ () => undefined }
+				onSelect={ mockOnSelect }
+			/>
+		);
+
+		expect( getSelectedTab() ).toHaveTextContent( 'Delta' );
+		expect( mockOnSelect ).toHaveBeenLastCalledWith( 'delta' );
+	} );
+
 	it( 'should disable the tab when `disabled` is true', async () => {
 		const user = setupUser();
 		const mockOnSelect = jest.fn();