SiblingInserter: Opening should clear block selection

… thus hiding any selected block's chrome.

editor/components/block-list/sibling-inserter.js

@@ -19,6 +19,9 @@ import {
 	isBlockInsertionPointVisible,
 	isBlockWithinSelection,
 } from '../../store/selectors';
+import {
+	clearSelectedBlock,
+} from '../../store/actions';
 
 class BlockListSiblingInserter extends Component {
 	constructor() {
@@ -35,6 +38,10 @@ class BlockListSiblingInserter extends Component {
 		// Prevent mouseout and blur while navigating the open inserter menu
 		// from causing the inserter to be unmounted.
 		this.setState( { isForcedVisible: isOpen } );
+
+		if ( isOpen ) {
+			this.props.clearSelectedBlock();
+		}
 	}
 
 	render() {
@@ -81,5 +88,8 @@ export default connect(
 				getBlockInsertionPoint( state ) === insertIndex
 			),
 		};
+	},
+	{
+		clearSelectedBlock,
 	}
 )( BlockListSiblingInserter );