Wp-calypso: Dashboard: Site Preview button does not load site if custom domain is used

[jerrysarcastic]

Noted this while helping a few site owners today. If a site is using a custom domain, when clicking the Site Preview button in the dashboard, no preview of the site is loaded, just a constantly spinning load icon.

Gif:

Note: preview seems to work fine if the site is using the default sitename.wordpress.com URL as primary, so only seems to affect sites with a custom domain (mapped or registered at WordPress.com) set to the primary address.

Also tested to see if site visibility is a factor, but preview will not load for either public or private sites.

[tyxla]

It appears we were previously forcing HTTPS for the preview frame, and I appear to have broken that with #13805. Fix coming right away.

[tyxla]

I've addressed this one in #13914, it could use a review and testing.

[tyxla]

@jerrysarcastic we've deployed the fix for this in #13914, could you please confirm this resolved the issue for you? Thanks!

[designsimply]

I tested with an AT site using a custom domain and a free plan and site preview is now working for me:

^{Seen at https://wordpress.com/themes/designsimply-automated-transfer-4.blog (click "Site Preview" in the W menu) using iPhone 6S Safari iOS 10.3.1 (Rebalance theme).}

^{Seen at https://wordpress.com/themes/designsimply-automated-transfer-4.blog (click "Site Preview" in the W menu) using Safari 10.1 on Mac OS X 10.12.4 (Rebalance theme).}

[jerrysarcastic]

Worked for me as well in testing a few wpcom and AT sites with custom domains. Thanks!