I just think it goes to show how little the window management of desktop OSes has improved over the years that desktop applications have had to up the ante...

I also think the differing behaviour between different apps implementing split panes (e.g. keyboard controls for creating/switching) is very annoying. Somtimes this flies in the face of any desktop's native window splitting or tab support as e.g. an app stops supporting multiple windows. For example, current browsers don't have a good way to configure usage without tabs, and at some point removed support for setting the window icon to the site's favicon.

Perhaps. Maybe iTerm's recent browser plugin, though useful, is a symptom of that lack of desktop UI progress.