Try improving the side UI for nested blocks

blocks/library/columns/editor.scss

@@ -0,0 +1,54 @@
+// These margins make sure that nested blocks stack/overlay with the parent block chrome
+// This is sort of an experiment at making sure the editor looks as much like the end result as possible
+// Potentially the rules here can apply to all nested blocks and enable stacking, in which case it should be moved elsewhere
+.wp-block-columns .editor-block-list__layout {
+	&:first-child {
+		margin-left: -$block-padding;
+	}
+	&:last-child {
+		margin-right: -$block-padding;
+	}
+
+	// This max-width is used to constrain the main editor column, it should not cascade into columns
+	.editor-block-list__block {
+		max-width: none;
+	}
+}
+
+// Wide: show no left/right margin on wide, so they stack with the column side UI
+.editor-block-list__block[data-align="wide"] .wp-block-columns .editor-block-list__layout {
+	&:first-child {
+		margin-left: 0;
+	}
+	&:last-child {
+		margin-right: 0;
+	}
+}
+
+// Fullwide: show margin left/right to ensure there's room for the side UI
+// This is not a 1:1 preview with the front-end where these margins would presumably be zero
+// @todo this could be revisited, by for example showing this margin only when the parent block was selected first
+// Then at least an unselected columns block would be an accurate preview
+.editor-block-list__block[data-align="full"] .wp-block-columns .editor-block-list__layout {
+	&:first-child {
+		margin-left: $block-side-ui-padding;
+	}
+	&:last-child {
+		margin-right: $block-side-ui-padding;
+	}
+}
+
+// Hide appender shortcuts in columns
+// @todo This essentially duplicates the mobile styles for the appender component
+// It would be nice to be able to use element queries in that component instead https://github.com/tomhodgins/element-queries-spec
+.wp-block-columns {
+	.editor-inserter-with-shortcuts {
+		display: none;
+	}
+
+	.editor-block-list__empty-block-inserter,
+	.editor-default-block-appender .editor-inserter {
+		left: auto;
+		right: $item-spacing;
+	}
+}

blocks/library/columns/index.js

@@ -15,6 +15,7 @@ import { RangeControl } from '@wordpress/components';
  * Internal dependencies
  */
 import './style.scss';
+import './editor.scss';
 import InspectorControls from '../../inspector-controls';
 import BlockControls from '../../block-controls';
 import BlockAlignmentToolbar from '../../block-alignment-toolbar';

blocks/library/paragraph/editor.scss

@@ -2,6 +2,11 @@
 	background: white;
 }
 
+// Don't show white background when a nesting parent is selected
+.editor-block-list__layout .editor-block-list__layout .editor-block-list__block .wp-block-paragraph {
+	background: inherit;
+}
+
 .blocks-font-size__main {
 	display: flex;
 	justify-content: space-between;

edit-post/assets/stylesheets/_variables.scss

@@ -46,6 +46,7 @@ $block-padding: 14px;
 $block-mover-margin: 18px;
 $block-spacing: 4px;
 $block-side-ui-padding: 36px;
+$block-side-ui-width: 28px;		// The side UI max height matches a single line of text, 56px. 28px is half, allowing 2 mover arrows
 
 // Buttons & UI Widgets
 $button-style__radius-roundrect: 4px;

edit-post/assets/stylesheets/_z-index.scss

@@ -33,6 +33,10 @@ $z-layers: (
 	'.components-drop-zone': 100,
 	'.components-drop-zone__content': 110,
 
+	// Block controls, particularly in nested contexts, floats aside block and
+	// should overlap most block content.
+	'.editor-block-list__block.is-{selected,hovered} .editor-block-{settings-menu,mover}': 100,
+
 	// Show sidebar above wp-admin navigation bar for mobile viewports:
 	// #wpadminbar { z-index: 99999 }
 	'.edit-post-sidebar': 100000,

edit-post/components/visual-editor/style.scss

@@ -47,8 +47,8 @@
 			margin-right: -$block-side-ui-padding;
 		}
 
-		&[data-align="full"] .editor-block-contextual-toolbar,
-		&[data-align="wide"] .editor-block-contextual-toolbar {
+		&[data-align="full"] > .editor-block-contextual-toolbar,
+		&[data-align="wide"] > .editor-block-contextual-toolbar { // don't affect nested block toolbars
 			max-width: $content-width + 2;	// 1px border left and right
 			margin-left: auto;
 			margin-right: auto;

editor/components/block-list/block.js

@@ -39,6 +39,7 @@ import InvalidBlockWarning from './invalid-block-warning';
 import BlockCrashWarning from './block-crash-warning';
 import BlockCrashBoundary from './block-crash-boundary';
 import BlockHtml from './block-html';
+import BlockBreadcrumb from './breadcrumb';
 import BlockContextualToolbar from './block-contextual-toolbar';
 import BlockMultiControls from './multi-controls';
 import BlockMobileToolbar from './block-mobile-toolbar';
@@ -438,11 +439,11 @@ export class BlockListBlock extends Component {
 
 		// Insertion point can only be made visible when the side inserter is
 		// not present, and either the block is at the extent of a selection or
-		// is the last block in the top-level list rendering.
+		// is the first block in the top-level list rendering.
 		const shouldShowInsertionPoint = (
-			( ! isMultiSelected && ! isLast ) ||
+			( ! isMultiSelected && ! isFirst ) ||
 			( isMultiSelected && isLastInSelection ) ||
-			( isLast && ! rootUID && ! isEmptyDefaultBlock )
+			( isFirst && ! rootUID && ! isEmptyDefaultBlock )
 		);
 
 		// Generate the wrapper class names handling the different states of the block.
@@ -479,6 +480,7 @@ export class BlockListBlock extends Component {
 			<IgnoreNestedEvents
 				ref={ this.setBlockListRef }
 				onMouseOver={ this.maybeHover }
+				onMouseOverHandled={ this.hideHoverEffects }
 				onMouseLeave={ this.hideHoverEffects }
 				className={ wrapperClassName }
 				data-type={ block.name }
@@ -493,6 +495,13 @@ export class BlockListBlock extends Component {
 				] }
 				{ ...wrapperProps }
 			>
+				{ shouldShowInsertionPoint && (
+					<BlockInsertionPoint
+						uid={ block.uid }
+						rootUID={ rootUID }
+						layout={ layout }
+					/>
+				) }
 				<BlockDropZone
 					index={ order }
 					rootUID={ rootUID }
@@ -514,6 +523,7 @@ export class BlockListBlock extends Component {
 						renderBlockMenu={ renderBlockMenu }
 					/>
 				) }
+				{ isHovered && <BlockBreadcrumb uid={ block.uid } /> }
 				{ shouldShowContextualToolbar && <BlockContextualToolbar /> }
 				{ isFirstMultiSelected && <BlockMultiControls rootUID={ rootUID } /> }
 				<IgnoreNestedEvents
@@ -561,13 +571,6 @@ export class BlockListBlock extends Component {
 					) }
 				</IgnoreNestedEvents>
 				{ !! error && <BlockCrashWarning /> }
-				{ shouldShowInsertionPoint && (
-					<BlockInsertionPoint
-						uid={ block.uid }
-						rootUID={ rootUID }
-						layout={ layout }
-					/>
-				) }
 				{ showSideInserter && (
 					<Fragment>
 						<div className="editor-block-list__side-inserter">

editor/components/block-list/breadcrumb.js

@@ -0,0 +1,77 @@
+/**
+ * WordPress dependencies
+ */
+import { compose } from '@wordpress/element';
+import { Dashicon, Tooltip, Toolbar, Button } from '@wordpress/components';
+import { withDispatch, withSelect } from '@wordpress/data';
+import { __ } from '@wordpress/i18n';
+
+/**
+ * Internal dependencies
+ */
+import NavigableToolbar from '../navigable-toolbar';
+import BlockTitle from '../block-title';
+
+/**
+ * Stops propagation of the given event argument. Assumes that the event has
+ * been completely handled and no other listeners should be informed.
+ *
+ * For the breadcrumb component, this is used for improved interoperability
+ * with the block's `onFocus` handler which selects the block, thus conflicting
+ * with the intention to select the root block.
+ *
+ * @param {Event} event Event for which propagation should be stopped.
+ */
+function stopPropagation( event ) {
+	event.stopPropagation();
+}
+
+/**
+ * Block breadcrumb component, displaying the label of the block. If the block
+ * descends from a root block, a button is displayed enabling the user to select
+ * the root block.
+ *
+ * @param {string}   props.uid             UID of block.
+ * @param {string}   props.rootUID         UID of block's root.
+ * @param {Function} props.selectRootBlock Callback to select root block.
+ *
+ * @return {WPElement} Breadcrumb element.
+ */
+function BlockBreadcrumb( { uid, rootUID, selectRootBlock } ) {
+	return (
+		<NavigableToolbar className="editor-block-breadcrumb">
+			<Toolbar>
+				{ rootUID && (
+					<Tooltip text={ __( 'Select parent block' ) }>
+						<Button
+							onClick={ selectRootBlock }
+							onFocus={ stopPropagation }
+						>
+							<Dashicon icon="arrow-left" uid={ uid } />
+						</Button>
+					</Tooltip>
+				) }
+				<BlockTitle uid={ uid } />
+			</Toolbar>
+		</NavigableToolbar>
+	);
+}
+
+export default compose( [
+	withSelect( ( select, ownProps ) => {
+		const { getBlockRootUID } = select( 'core/editor' );
+		const { uid } = ownProps;
+
+		return {
+			rootUID: getBlockRootUID( uid ),
+		};
+	} ),
+	withDispatch( ( dispatch, ownProps ) => {
+		const { rootUID } = ownProps;
+		const { selectBlock } = dispatch( 'core/editor' );
+
+		return {
+			selectRootBlock: () => selectBlock( rootUID ),
+		};
+	} ),
+] )( BlockBreadcrumb );

editor/components/block-list/ignore-nested-events.js

@@ -43,10 +43,7 @@ class IgnoreNestedEvents extends Component {
 	 * @return {void}
 	 */
 	proxyEvent( event ) {
-		// Skip if already handled (i.e. assume nested block)
-		if ( event.nativeEvent._blockHandled ) {
-			return;
-		}
+		const isHandled = !! event.nativeEvent._blockHandled;
 
 		// Assign into the native event, since React will reuse their synthetic
 		// event objects and this property assignment could otherwise leak.
@@ -55,7 +52,14 @@ class IgnoreNestedEvents extends Component {
 		event.nativeEvent._blockHandled = true;
 
 		// Invoke original prop handler
-		const propKey = this.eventMap[ event.type ];
+		let propKey = this.eventMap[ event.type ];
+
+		// If already handled (i.e. assume nested block), only invoke a
+		// corresponding "Handled"-suffixed prop callback.
+		if ( isHandled ) {
+			propKey += 'Handled';
+		}
+
 		if ( this.props[ propKey ] ) {
 			this.props[ propKey ]( event );
 		}
@@ -69,16 +73,24 @@ class IgnoreNestedEvents extends Component {
 			...Object.keys( props ),
 		], ( result, key ) => {
 			// Try to match prop key as event handler
-			const match = key.match( /^on([A-Z][a-zA-Z]+)$/ );
+			const match = key.match( /^on([A-Z][a-zA-Z]+?)(Handled)?$/ );
 			if ( match ) {
+				const isHandledProp = !! match[ 2 ];
+				if ( isHandledProp ) {
+					// Avoid assigning through the invalid prop key. This
+					// assumes mutation of shallow clone by above spread.
+					delete props[ key ];
+				}
+
 				// Re-map the prop to the local proxy handler to check whether
 				// the event has already been handled.
-				result[ key ] = this.proxyEvent;
+				const proxiedPropName = 'on' + match[ 1 ];
+				result[ proxiedPropName ] = this.proxyEvent;
 
 				// Assign event -> propName into an instance variable, so as to
 				// avoid re-renders which could be incurred either by setState
 				// or in mapping values to a newly created function.
-				this.eventMap[ match[ 1 ].toLowerCase() ] = key;
+				this.eventMap[ match[ 1 ].toLowerCase() ] = proxiedPropName;
 			}
 
 			return result;

editor/components/block-list/insertion-point.js

@@ -60,7 +60,7 @@ export default compose(
 	connect(
 		( state, { uid, rootUID } ) => {
 			const blockIndex = uid ? getBlockIndex( state, uid, rootUID ) : -1;
-			const insertIndex = blockIndex + 1;
+			const insertIndex = blockIndex;
 			const insertionPoint = getBlockInsertionPoint( state );
 			const block = uid ? getBlock( state, uid ) : null;
 

editor/components/block-list/layout.js

@@ -24,7 +24,6 @@ import { Component } from '@wordpress/element';
  */
 import './style.scss';
 import BlockListBlock from './block';
-import BlockInsertionPoint from './insertion-point';
 import IgnoreNestedEvents from './ignore-nested-events';
 import DefaultBlockAppender from '../default-block-appender';
 import {
@@ -216,7 +215,6 @@ class BlockListLayout extends Component {
 
 		return (
 			<div className={ classes }>
-				{ !! blockUIDs.length && <BlockInsertionPoint rootUID={ rootUID } layout={ defaultLayout } /> }
 				{ map( blockUIDs, ( uid, blockIndex ) => (
 					<BlockListBlock
 						key={ 'block-' + uid }