What Problem This Solves

The Control UI's desktop chrome is a thick left sidebar that stacks brand, pinned navigation, a "More" section, a New session button, the session list, and a footer icon row. Most of that column is chrome rather than content, and the cost doubles in chat split view where horizontal space matters most. Two smaller papers cut the same surface:

• In split view, the collapsed session-workspace strip renders a document glyph that looks like a button but is a dead, non-interactive span (chat-workspace-rail__collapsed-icon), sitting directly under a working button.
• The session-workspace rail is hard-fixed to the right edge of each pane. The operator terminal already supports right/bottom docking, but the rail that is conceptually "attached to the session" does not.

Proposed Change

• Move app chrome into a slim persistent topbar: brand + connection dot, Chat + pinned navigation pills, a More menu (remaining destinations, plugin tabs, Docs, pin editor), command-palette search, pairing, theme toggle, Settings.
• Slim the left column to a sessions-only panel; Cmd+B hides it entirely (the 78px icon rail becomes obsolete since navigation lives in the topbar).
• Replace the fixed split-view toolbar (which mirrored pane geometry with scroll-sync) with in-flow per-pane headers.
• Make the session workspace rail dockable right or bottom inside its pane, via drag on its header or a dock button, mirroring the terminal panel; make the collapsed strip's file glyph a real button with a changed-file count.
• Keep the narrow-viewport drawer behavior; keep Mac app drag-region geometry compatible.

User Impact

More horizontal room for content everywhere, a calmer first screen, and a session panel that reads as content instead of chrome. Split view gets honest per-pane title bars. The dead button becomes a working affordance.