The trigger for button visibility and the button being active seem to sometimes not be in sync

I've observed on a chrome-based browser that at a window width of about 1000px, as I scroll down the page. The button is visible, but not clickable for some time. In my particular case, that is until the button is quite close to the bottom of the page, but my assumption is that if the page was longer, it would be clickable farther from the bottom of the page.