Save meta on preview

docs/data/data-core-editor.md

@@ -1058,6 +1058,18 @@ Returns true if the post is autosaving, or false otherwise.
 
 Whether the post is autosaving.
 
+### isPreviewingPost
+
+Returns true if the post is being previewed, or false otherwise.
+
+*Parameters*
+
+ * state: Global application state.
+
+*Returns*
+
+Whether the post is being previewed.
+
 ### getSuggestedPostFormat
 
 Returns a suggested post format for the current post, inferred only if there

packages/edit-post/src/store/effects.js

@@ -45,24 +45,27 @@ const effects = {
 
 		let wasSavingPost = select( 'core/editor' ).isSavingPost();
 		let wasAutosavingPost = select( 'core/editor' ).isAutosavingPost();
+		let wasPreviewingPost = select( 'core/editor' ).isPreviewingPost();
 		// Save metaboxes when performing a full save on the post.
 		subscribe( () => {
 			const isSavingPost = select( 'core/editor' ).isSavingPost();
 			const isAutosavingPost = select( 'core/editor' ).isAutosavingPost();
+			const isPreviewingPost = select( 'core/editor' ).isPreviewingPost();
 			const hasActiveMetaBoxes = select( 'core/edit-post' ).hasMetaBoxes();
 
-			// Save metaboxes on save completion when past save wasn't an autosave.
+			// Save metaboxes on save completion, except for autosaves that are not a post preview.
 			const shouldTriggerMetaboxesSave = (
 				hasActiveMetaBoxes &&
 				wasSavingPost &&
-				! wasAutosavingPost &&
+				( ! wasAutosavingPost || wasPreviewingPost ) &&
 				! isSavingPost &&
-				! isAutosavingPost
+				( ! isAutosavingPost || isPreviewingPost )
 			);
 
 			// Save current state for next inspection.
 			wasSavingPost = isSavingPost;
 			wasAutosavingPost = isAutosavingPost;
+			wasPreviewingPost = isPreviewingPost;
 
 			if ( shouldTriggerMetaboxesSave ) {
 				store.dispatch( requestMetaBoxUpdates() );

packages/editor/src/components/post-preview-button/index.js

@@ -151,7 +151,7 @@ export class PostPreviewButton extends Component {
 
 		// Request an autosave. This happens asynchronously and causes the component
 		// to update when finished.
-		this.props.autosave();
+		this.props.autosave( { isPreview: true } );
 
 		// Display a 'Generating preview' message in the Preview tab while we wait for the
 		// autosave to finish.

packages/editor/src/store/selectors.js

@@ -1487,6 +1487,17 @@ export function isAutosavingPost( state ) {
 	return isSavingPost( state ) && state.saving.isAutosave;
 }
 
+/**
+ * Returns true if the post is being previewed, or false otherwise.
+ *
+ * @param {Object} state Global application state.
+ *
+ * @return {boolean} Whether the post is being previewed.
+ */
+export function isPreviewingPost( state ) {
+	return isSavingPost( state ) && state.saving.isPreview;
+}
+
 /**
  * Returns a suggested post format for the current post, inferred only if there
  * is a single block within the post and it is of a type known to match a