Preview button does not redirect if pressed while unsaved changes exist

[aduth]

Related: #7124
Separate from publish preview task (#4099)

If the user hits the Preview button for a draft where unsaved changes exist, the preview window appears but the user is never redirected to the post.

Steps to reproduce:

1. Navigate to Posts > Add New
2. Enter a title
3. Hit Preview button (before autosave occurs)

Expected result:

Intermediary page appears, but eventually redirects to preview.

Actual result:

Redirect never occurs.

Workaround:

If the user first hits Save before Preview, then the preview loads correctly.

Suspected cause:

The logic for detecting whether a redirect should take place depends on the post's modified field being updated. This does not occur for autosaves. A preview is expected to be saved as an autosave (at least for published posts, which are not expected to be updated immediately).

gutenberg/editor/components/post-preview-button/index.js

Lines 28 to 36 in e86d7b6

	const hasFinishedSaving = (
	isAwaitingSave &&
	modified !== this.props.modified
	);
	if ( hasFinishedSaving && this.previewWindow ) {
	this.previewWindow.location = link;
	this.setState( { isAwaitingSave: false } );
	}

[jenny-leung]

I have the same issue with Firefox and Chrome. Both do not load preview if not saved.
Difference:
Chrome opens a new tab with "about:blank" with no console log error.
Firefox shows the same url as the post edit page instead of the preview url with the following console log error every time I click preview without saving draft
TypeError: this.previewWindow is undefined[Learn More] index.js:12:150551
If I do not save draft and open the preview link from browser history instead of clicking the preview button, the preview renders correctly. However, every time I click preview, it changes the url. It doesn't keep the working url and refresh.

[aduth]

Should be fixed in #7130