Update heading block behaviour on ENTER key to create empty paragraph…

… instead of heading (WordPress#10326)

The onSplit method for the heading block will now replace an empty heading block with an empty paragraph block for the 'before' content.

This allows the ENTER key pressed at the beginning of a heading to create an empty paragraph above it. Also, when the ENTER key is pressed at the end of a heading, an empty paragraph block is created below, instead of another heading block. The RichText component now also calls event.stopImmediatePropagation() before splitContent() to prevent TinyMCE from throwing an error. This avoids a similar error as in WordPress#4580.

packages/block-library/src/heading/edit.js

@@ -52,10 +52,20 @@ export default function HeadingEdit( {
 					insertBlocksAfter ?
 						( before, after, ...blocks ) => {
 							setAttributes( { content: before } );
-							insertBlocksAfter( [
-								...blocks,
-								createBlock( 'core/paragraph', { content: after } ),
-							] );
+							if ( ! before.text && after.text ) {
+								onReplace( createBlock( 'core/paragraph', { content: before } ) );
+							}
+							if ( after.text ) {
+								insertBlocksAfter( [
+									...blocks,
+									createBlock( 'core/heading', { content: after } ),
+								] );
+							} else {
+								insertBlocksAfter( [
+									...blocks,
+									createBlock( 'core/paragraph', { content: after } ),
+								] );
+							}
 						} :
 						undefined
 				}

packages/editor/src/components/rich-text/index.js

@@ -610,6 +610,11 @@ export class RichText extends Component {
 			} else if ( event.shiftKey || ! this.onSplit ) {
 				this.onChange( insertLineBreak( record ) );
 			} else {
+				// Some implementations of onSplit might also call onReplace, and
+				// calling onReplace() will destroy the editor, so it's
+				// important that we stop other handlers (e.g. ones
+				// registered by TinyMCE) from also handling this event.
+				event.stopImmediatePropagation();
 				this.splitContent();
 			}
 		}