First subagent gets a horizontal 60/40 split (main left, agent right). Subsequent subagents split vertically within the right column so they stack. rightColumnPanes tracks right-column pane order so each new split targets the last pane in the stack.
First subagent gets a horizontal 60/40 split (main left, agent right). Subsequent subagents split vertically within the right column so they stack. rightColumnPanes tracks right-column pane order so each new split targets the last pane in the stack.