I’ve noticed that with any e-commerce site I create using the Shopify Buy Button in conjunction with a page loader, the page shows briefly before my loading screen, the loader then shows up for a second, then disappears as it should, but not until the ugliness of the page has already been exposed without the products having been loaded yet.
This will show you what I am talking about.
If you check out any of the other pages on the site, the loader works as expected. This is the case on any Shopify related site I have designed using the Buy Button. I imagine it’s something in the way Shopify has the hierarchy of the code set up, but I don’t really know enough about that to fully understand or do anything about it.