As a direct result of your feedback, ACR 8.5 has a new button designed to display a per-panel preview that is applied directly to the main view of the image! Clicking this button will reset the settings in the selected panel to their defaults. Clicking it again will reset them to the previous settings. Or, you can use the shortcut Command + Option + P (Mac) | Control + Alt + P (Win) to toggle the preview.
Although the preview behavior might appear to look the same as it did in previous versions, this new button actually works a bit differently “under the hood”. Instead of simply showing and hiding the settings in a panel, this button actually resets the panel (clicking the button again restores the previous settings).
So, you might be asking why did we change the per-panel preview behavior? Well, since Camera Raw is not a database program (like Lightroom is), it can’t keep track of different “states” that a panel might be in. This means that in previous versions of Camera Raw, if you had toggled off the preview state of a panel, and then clicked “Done” or “Open Image”, Camera Raw would apply the slider values—even if the preview was turned off for that panel. Therefore, what you saw in Camera Raw may not have matched the resulting file. As you can imagine, when this mismatch occurred, it was not only confusing to the customer, but also unacceptable to the engineering team.
With this release, I believe the engineers have provided us with the best of both worlds; we can still use the new Before/After features (those are completely unchanged), as well as have an improved per-panel preview as a standalone feature.