I've been working on search UI. This search UI is substantially more complex than the average one you might come across. There are potentially multiple search

I've been working on search UI. This search UI is substantially more complex than the average one you might come across. There are potentially multiple search bars (added on command), to which many different variants of filters and search terms can be added. They provide an aggregate search expression which is not reducible to boolean logic. The search results update interactively when the search terms are altered. It's specialized tool, and meant to be used by people of a certain vocation rather than the general public. The reason that the search results update interactively is to provide a short feedback loop on the effect of the search terms. Building a mental model ahead of time for the interactions between the query and the hundreds of millions of potential results is a tall order. It therefore seems sensible to provide affordances which facilitate exploration and (in some sense) experimentation. To this end, search results also expand inline to provide a way to inspect results quickly, with the purpose of validating whether the search terms were effective or need alteration. Now, where do I draw the line between what counts as a "new page" or the "same page" with this? Do I reload the page on every alteration of the search terms? Do I scrap automatic reloading between alterations of search terms and only do so explicitly when the user clicks a button? Should it navigate to a new page when interacting with search results rather than previewing them inline? This is all rhetorical, of course. All of those changes would make for a much poorer experience for this particular tool and the particular target user group. Today the web is used to build a lot of stuff where "page navigation" is a poor model to work with as a basis. I can see there being a case for it where a "page" really is a self-contained unit of information. But there are also a lot of cases where declaring some particular state in a long user journey of interconnected actions as a "different page" is going to be arbitrary.