Insert Chat and Zen immediately before the Git utility group in normal mode, then relocate them to the conversation root while Zen hides the header. Collapse the composer with measurement-safe clipping instead of display none to prevent incorrect empty textarea height after restoring it.
Mount Chat and Zen controls on the conversation root at top-right so they cannot overlap the scroll-to-bottom affordance. Hide the semantic conversation header in Zen mode and keep the exit control independent from composer visibility.
Keep hidden sidebar and details columns participating in CSS Grid so the center conversation remains in the middle track. Hide their contents with visibility and disable interactions while removing only resize handles from layout.
Hide workspace and agent content when the native sidebar rail is collapsed. Add persistent whole-composer collapse controls and a temporary Zen mode that removes side panels, chat header, and composer while keeping an in-chat exit control available. Cover state transitions, Escape handling, React rerender idempotence, and cleanup with a dependency-free DOM regression test.
