Google crawls websites at a fixed size and in a fixed state. What does this mean?
Fixed size. Googlebot won’t zoom, scroll, or resize the screen during a crawl. Because of this, Googlebot loads the website at a larger height, which allows its bot to capture as much of the page as possible. Even still, on longer pages, Googlebot may miss out on certain content contained on the page.
Fixed state. Human visitors can change the state of the page during their visit by clicking on items, pressing keys, interacting with form fields, and more. Googlebot doesn’t do this. Once Googlebot loads a website, it doesn’t interact and will only see the website as it first appears. This could be a problem for websites that need these interactions to show all their content.
Fixed size rarely presents a problem. If your page is longer than 9K pixels tall, things should be reworked to make sure Googlebot can see everything on the page. (Honestly, that long of a page probably isn’t great for visitors either.)
Fixed state can create problems for websites that rely heavily on JavaScript. For example, infinite scroll may be used and would change the state of the page when visitors scroll. If you don’t scroll the page (like bots), you’ll have a very different experience and will likely miss out on content. It is important to build alternative means of accessing content that do not require visitors to interact with the page.