We should probably check if usedPostId is defined. Otherwise, you'll see the null flash in the title when visiting wp-admin/site-editor.php, because the editor is resolving the edited template from the server, and we get a false positive.

const _isLoaded =
				usedPostId &&
				hasFinishedResolution( 'getEditedEntityRecord', [
					'postType',
					usedPostType,
					usedPostId,
				] );

Ah, of course, good catch! Added in d032f4d

I don't understand why the extra usedPostId && ... check was needed here. If usedPostId is null, the hasFinishedSelector returns true for the null value?

I'd say the idea is to not call the hasFinishedResolution selector at all if the post ID is null since we already know the result.

There's a point where both selectors use undefined as postId. The getEditedEntityRecord will resolve and return an empty object, and hasFinishedResolution will report that our template was loaded.

Example:

const { testRecord, testResolved } = useSelect( ( select ) => {
	const { getEditedEntityRecord, hasFinishedResolution } =
		select( coreStore );

	return {
		testRecord: getEditedEntityRecord(
			'postType',
			'wp_template',
			undefined
		),
		testResolved: hasFinishedResolution( 'getEditedEntityRecord', [
			'postType',
			'wp_template',
			undefined,
		] ),
	};
} );

console.log( { testRecord, testResolved } )

Oh, now I see what's happening. A getEntityRecord( 'postType', 'wp_template', undefined ) call doesn't make sense. It will do a lookup like return items[ key ], but there is never a template with id === undefined, so it will always return undefined.

But it still calls the resolver, where undefined key does make sense. Instead of fetching a specific template from /templates/{key}, it will fetch all templates from /templates and store the list.

That's why we end up in a situation where getEntityRecord returns undefined (there's no item with undefined key) and hasFinishedResolution returns true (the list is loaded).

If usedPostId is not known, the hook shouldn't call any selector at all and return an empty-ish object right away.