· Browser
The browser pane, and browser agents
A real browser as a pane kind, drivable by an agent, laid out without an overlay so it obeys the same split grammar as everything else.
An agent that can read a page is a different agent from one that can only be told about it. The browser pane makes the page a first-class object in the workspace rather than a screenshot pasted into a prompt.
It is laid out without an overlay, which is what lets it split, resize and persist like a session or a terminal instead of floating above the grid pretending to.
Browser agents drive it through the same tool surface a person drives it with (navigate, snapshot, act by reference, read, screenshot), so what the agent did is reconstructable afterwards.