Customer testing prior production

Do your customers have a look at new functionality before it goes to production? Or does it just get shipped out to them?

I have very little experience in the browser app space thus far, but normally yes.
I’ve always had “materiel” mockups that were shown to a customer, sometimes 3d printed parts let a customer see the button layout even if the buttons don’t actually work, to validate ergonomics. Ergonomics on IOS for one handed use for example: is terrible unless you hold the phone/tablet from the top, because “furniture” or common buttons are displayed at the top!
For pure software that should be no different, but normally it’s only one user who gets to see the mock-ups and give feedback before we start building the thing.

I have once worked on a UX centric product rewrite where the customer did not see the layouts first. We were porting a native console to work in a browser. But we used professionals to design it and ran loads of tiny internal “speed-test” trials of the UI to get the workflows optimized. In the end we did pretty good on the porting of it, it intentionally looked very similar to the old one though.

So my experience is that you often do, but I am pretty sure that even Microsoft demoed the new “Metro” UI to users first. But that did not guarantee that the idea that first saw light in 2003 and came out in Windows 8 was still going to really still be hip and adopted by all in 2020. Although that fail was more down to implementation than the functionality IMHO.

We put the product in the hand of different external stakeholders throughout the production well before it is the final product. For us it’s not so much testing as a marketing teaser / opportunity for feedback before finalizing the product.