Block List: Extract scroll preservation as non-visual component

edit-post/components/layout/index.js

@@ -16,6 +16,7 @@ import {
 	EditorNotices,
 	PostPublishPanel,
 	DocumentTitle,
+	PreserveScrollInReorder,
 } from '@wordpress/editor';
 
 /**
@@ -83,6 +84,7 @@ function Layout( {
 			<Header />
 			<div className="edit-post-layout__content" role="region" aria-label={ __( 'Editor content' ) } tabIndex="-1">
 				<EditorNotices />
+				<PreserveScrollInReorder />
 				<div className="edit-post-layout__editor">
 					<EditorModeKeyboardShortcuts />
 					{ mode === 'text' && <TextEditor /> }

editor/components/block-list/block.js

@@ -13,7 +13,6 @@ import { Component, findDOMNode, compose } from '@wordpress/element';
 import {
 	keycodes,
 	focus,
-	getScrollContainer,
 	placeCaretAtHorizontalEdge,
 	placeCaretAtVerticalEdge,
 } from '@wordpress/utils';
@@ -100,8 +99,6 @@ export class BlockListBlock extends Component {
 		this.onClick = this.onClick.bind( this );
 		this.selectOnOpen = this.selectOnOpen.bind( this );
 		this.onSelectionChange = this.onSelectionChange.bind( this );
-
-		this.previousOffset = null;
 		this.hadTouchStart = false;
 
 		this.state = {
@@ -140,31 +137,12 @@ export class BlockListBlock extends Component {
 	}
 
 	componentWillReceiveProps( newProps ) {
-		if (
-			this.props.order !== newProps.order &&
-			( newProps.isSelected || newProps.isFirstMultiSelected )
-		) {
-			this.previousOffset = this.node.getBoundingClientRect().top;
-		}
-
 		if ( newProps.isTyping || newProps.isSelected ) {
 			this.hideHoverEffects();
 		}
 	}
 
 	componentDidUpdate( prevProps ) {
-		// Preserve scroll prosition when block rearranged
-		if ( this.previousOffset ) {
-			const scrollContainer = getScrollContainer( this.node );
-			if ( scrollContainer ) {
-				scrollContainer.scrollTop = scrollContainer.scrollTop +
-					this.node.getBoundingClientRect().top -
-					this.previousOffset;
-			}
-
-			this.previousOffset = null;
-		}
-
 		// Bind or unbind mousemove from page when user starts or stops typing
 		if ( this.props.isTyping !== prevProps.isTyping ) {
 			if ( this.props.isTyping ) {
@@ -201,8 +179,7 @@ export class BlockListBlock extends Component {
 
 	bindBlockNode( node ) {
 		// Disable reason: The block element uses a component to manage event
-		// nesting, but we rely on a raw DOM node for focusing and preserving
-		// scroll offset on move.
+		// nesting, but we rely on a raw DOM node for focusing.
 		//
 		// eslint-disable-next-line react/no-find-dom-node
 		this.node = findDOMNode( node );

editor/components/index.js

@@ -65,6 +65,7 @@ export { default as Inserter } from './inserter';
 export { default as MultiBlocksSwitcher } from './block-switcher/multi-blocks-switcher';
 export { default as MultiSelectScrollIntoView } from './multi-select-scroll-into-view';
 export { default as NavigableToolbar } from './navigable-toolbar';
+export { default as PreserveScrollInReorder } from './preserve-scroll-in-reorder';
 export { default as Warning } from './warning';
 export { default as WritingFlow } from './writing-flow';
 

editor/components/multi-select-scroll-into-view/index.js

@@ -10,6 +10,11 @@ import { Component } from '@wordpress/element';
 import { withSelect } from '@wordpress/data';
 import { getScrollContainer } from '@wordpress/utils';
 
+/**
+ * Internal dependencies
+ */
+import { getBlockDOMNode } from '../../utils/dom';
+
 class MultiSelectScrollIntoView extends Component {
 	componentDidUpdate() {
 		// Relies on expectation that `componentDidUpdate` will only be called
@@ -29,7 +34,7 @@ class MultiSelectScrollIntoView extends Component {
 			return;
 		}
 
-		const extentNode = document.querySelector( '[data-block="' + extentUID + '"]' );
+		const extentNode = getBlockDOMNode( extentUID );
 		if ( ! extentNode ) {
 			return;
 		}

editor/components/preserve-scroll-in-reorder/index.js

@@ -0,0 +1,80 @@
+/**
+ * WordPress dependencies
+ */
+import { Component } from '@wordpress/element';
+import { withSelect } from '@wordpress/data';
+import { getScrollContainer } from '@wordpress/utils';
+
+/**
+ * Internal dependencies
+ */
+import { getBlockDOMNode } from '../../utils/dom';
+
+/**
+ * Non-visual component which preserves offset of selected block within nearest
+ * scrollable container while reordering.
+ *
+ * @example
+ *
+ * ```jsx
+ * <PreserveScrollInReorder />
+ * ```
+ */
+class PreserveScrollInReorder extends Component {
+	componentWillUpdate( nextProps ) {
+		const { blockOrder, selectionStart } = nextProps;
+		if ( blockOrder !== this.props.blockOrder && selectionStart ) {
+			this.setPreviousOffset( selectionStart );
+		}
+	}
+
+	componentDidUpdate() {
+		if ( this.previousOffset ) {
+			this.restorePreviousOffset();
+		}
+	}
+
+	/**
+	 * Given the block UID of the start of the selection, saves the block's
+	 * top offset as an instance property before a reorder is to occur.
+	 *
+	 * @param {string} selectionStart UID of selected block.
+	 */
+	setPreviousOffset( selectionStart ) {
+		const blockNode = getBlockDOMNode( selectionStart );
+		if ( ! blockNode ) {
+			return;
+		}
+
+		this.previousOffset = blockNode.getBoundingClientRect().top;
+	}
+
+	/**
+	 * After a block reordering, restores the previous viewport top offset.
+	 */
+	restorePreviousOffset() {
+		const { selectionStart } = this.props;
+		const blockNode = getBlockDOMNode( selectionStart );
+		if ( blockNode ) {
+			const scrollContainer = getScrollContainer( blockNode );
+			if ( scrollContainer ) {
+				scrollContainer.scrollTop = scrollContainer.scrollTop +
+					blockNode.getBoundingClientRect().top -
+					this.previousOffset;
+			}
+		}
+
+		delete this.previousOffset;
+	}
+
+	render() {
+		return null;
+	}
+}
+
+export default withSelect( ( select ) => {
+	return {
+		blockOrder: select( 'core/editor' ).getBlockOrder(),
+		selectionStart: select( 'core/editor' ).getBlockSelectionStart(),
+	};
+} )( PreserveScrollInReorder );

editor/store/selectors.js

@@ -464,6 +464,32 @@ export function getBlockCount( state, rootUID ) {
 	return getBlockOrder( state, rootUID ).length;
 }
 
+/**
+ * Returns the current block selection start. This value may be null, and it
+ * may represent either a singular block selection or multi-selection start.
+ * A selection is singular if its start and end match.
+ *
+ * @param {Object} state Global application state.
+ *
+ * @return {?string} UID of block selection start.
+ */
+export function getBlockSelectionStart( state ) {
+	return state.blockSelection.start;
+}
+
+/**
+ * Returns the current block selection end. This value may be null, and it
+ * may represent either a singular block selection or multi-selection end.
+ * A selection is singular if its start and end match.
+ *
+ * @param {Object} state Global application state.
+ *
+ * @return {?string} UID of block selection end.
+ */
+export function getBlockSelectionEnd( state ) {
+	return state.blockSelection.end;
+}
+
 /**
  * Returns the number of blocks currently selected in the post.
  *

editor/utils/dom.js

@@ -0,0 +1,12 @@
+/**
+ * Given a block UID, returns the corresponding DOM node for the block, if
+ * exists. As much as possible, this helper should be avoided, and used only
+ * in cases where isolated behaviors need remote access to a block node.
+ *
+ * @param {string} uid Block UID.
+ *
+ * @return {Element} Block DOM node.
+ */
+export function getBlockDOMNode( uid ) {
+	return document.querySelector( '[data-block="' + uid + '"]' );
+}