Enable experiencing a page, solution or site as a different user, e.g. provide the ability to impersonate a user with fewer rights
As Microsoft increasingly supports tailored experiences, we need the ability to operate as a user with equivalent or fewer credentials, including just not being the person who added information.
For example, a site owner who made updates to a site is seeing Updated Pages (with explicit pages underneath) in the left navigation, but has no rights to remove the navigation and didn't realize that other people weren't seeing those pages.
Another case was where the SCA had no issue opening a document with embedded documents, but it would not open for site owners and contributors.
In our on-prem environment, I was able to use a test account and assign it with certain rights to verify that those users were going to get the experience I intended, and I was able to do that with a test account. In the SharePoint Online world, for an out-of-the-box solution I'd need a second licensed account for testing in production.
Checking permissions to a site, list, library or item helps, but just isn't enough for testing the experience on a page in or in a solution.
A potential way to implement this would be to allow me to click on the name I'm logged in as, and in addition to About Me, Sign Out and Personalize this Page, have an option to impersonate a specific user (who has fewer or equivalent rights to me) or a certain level of rights (equivalent to or less than the rights I have).
Some developers in teams help test each other's solutions, changing who has what rights, but for those of us who largely work independently, we don't have that luxury.
Ian Caldwell commented
Yes, this feature is needed - or at the very least an option to turn-off edit mode so that we can see the current live version of a page