if thre is only one there is only one

packages/block-editor/src/components/inserter/index.js

@@ -1,11 +1,21 @@
+/**
+ * External dependencies
+ */
+import classnames from 'classnames';
+import {
+	get,
+} from 'lodash';
 /**
  * WordPress dependencies
  */
 import { __ } from '@wordpress/i18n';
-import { Dropdown, IconButton } from '@wordpress/components';
+import { Dropdown, IconButton, Button, Icon } from '@wordpress/components';
 import { Component } from '@wordpress/element';
-import { withSelect } from '@wordpress/data';
+import { withDispatch, withSelect } from '@wordpress/data';
 import { compose, ifCondition } from '@wordpress/compose';
+import {
+	createBlock,
+} from '@wordpress/blocks';
 
 /**
  * Internal dependencies
@@ -83,7 +93,23 @@ class Inserter extends Component {
 	}
 
 	render() {
-		const { position } = this.props;
+		const { position, hasOneAllowedItem, createIfOne } = this.props;
+		let toggle = null;
+		if ( hasOneAllowedItem ) {
+			toggle = () => {
+				return (
+					<Button
+						className={ classnames( 'block-editor-button-block-appender' ) }
+						onClick={ createIfOne }
+					>
+						<span className="screen-reader-text">{ __( 'Add Block' ) }</span>
+						<Icon icon="insert" />
+					</Button>
+				);
+			};
+		} else {
+			toggle = this.renderToggle;
+		}
 
 		return (
 			<Dropdown
@@ -93,7 +119,7 @@ class Inserter extends Component {
 				onToggle={ this.onToggle }
 				expandOnMobile
 				headerTitle={ __( 'Add a block' ) }
-				renderToggle={ this.renderToggle }
+				renderToggle={ toggle }
 				renderContent={ this.renderContent }
 			/>
 		);
@@ -102,10 +128,62 @@ class Inserter extends Component {
 
 export default compose( [
 	withSelect( ( select, { rootClientId } ) => {
-		const { hasInserterItems } = select( 'core/block-editor' );
+		const { hasInserterItems, hasOneAllowedItem } = select( 'core/block-editor' );
 
 		return {
 			hasItems: hasInserterItems( rootClientId ),
+			hasOneAllowedItem: hasOneAllowedItem( rootClientId ),
+		};
+	} ),
+	withDispatch( ( dispatch, ownProps, { select } ) => {
+		// eslint-disable-next-line no-restricted-syntax
+		function getInsertionIndex() {
+			const {
+				getBlockIndex,
+				getBlockSelectionEnd,
+				getBlockOrder,
+			} = select( 'core/block-editor' );
+			const { clientId, destinationRootClientId, isAppender } = ownProps;
+
+			// If the clientId is defined, we insert at the position of the block.
+			if ( clientId ) {
+				return getBlockIndex( clientId, destinationRootClientId );
+			}
+
+			// If there a selected block, we insert after the selected block.
+			const end = getBlockSelectionEnd();
+			if ( ! isAppender && end ) {
+				return getBlockIndex( end, destinationRootClientId ) + 1;
+			}
+
+			// Otherwise, we insert at the end of the current rootClientId
+			return getBlockOrder( destinationRootClientId ).length;
+		}
+		const { rootClientId } = ownProps;
+
+		// eslint-disable-next-line no-restricted-syntax
+		function createIfOne( ) {
+			const {
+				insertBlock,
+			} = dispatch( 'core/block-editor' );
+			const {
+				getBlockListSettings,
+			} = select( 'core/block-editor' );
+			const parentBlockListSettings = getBlockListSettings( rootClientId );
+			const parentAllowedBlocks = get( parentBlockListSettings, [ 'allowedBlocks' ] );
+			if ( parentAllowedBlocks.length > 1 ) {
+				return false;
+			}
+			const insertedBlock = createBlock( parentAllowedBlocks[ 0 ] );
+			insertBlock(
+				insertedBlock,
+				getInsertionIndex(),
+				rootClientId
+			);
+		}
+
+		return {
+			createIfOne,
 		};
 	} ),
 	ifCondition( ( { hasItems } ) => hasItems ),

packages/block-editor/src/store/selectors.js

@@ -1328,6 +1328,29 @@ export const hasInserterItems = createSelector(
 	],
 );
 
+/**
+ * Determines whether there is only one item that may be inserted.
+ * @param {Object}  state        Editor state.
+ * @param {?string} rootClientId Optional root client ID of block list.
+ *
+ * @return {boolean} True if there is one item available, false if zero or more than one.
+ */
+export const hasOneAllowedItem = createSelector(
+	( state, rootClientId = null ) => {
+		if ( rootClientId ) {
+			const parentBlockListSettings = getBlockListSettings( state, rootClientId );
+			return ( !! get( parentBlockListSettings, [ 'allowedBlocks' ] ) &&
+				get( parentBlockListSettings, [ 'allowedBlocks' ] ).length === 1 );
+		}
+
+		return false;
+	},
+	( state, rootClientId ) => [
+		state,
+		rootClientId,
+	],
+);
+
 /**
  * Returns the Block List settings of a block, if any exist.
  *