feat: add reversible workspace panels

crates/lumbridge-core/src/workspace.rs

@@ -268,6 +268,13 @@ pub enum WorkspaceCommand {
         ratio: SplitRatio,
         placement: PanePlacement,
     },
+    AttachPane {
+        target: PaneId,
+        pane: PaneId,
+        axis: SplitAxis,
+        ratio: SplitRatio,
+        placement: PanePlacement,
+    },
     SelectPane {
         pane: PaneId,
     },
@@ -291,6 +298,7 @@ impl WorkspaceCommand {
             } => WorkspaceCapability::Execute,
             Self::SplitPane { pane, .. } if pane.launch.is_some() => WorkspaceCapability::Execute,
             Self::SplitPane { .. }
+            | Self::AttachPane { .. }
             | Self::SelectPane { .. }
             | Self::RenamePane { .. }
             | Self::ClosePane {
@@ -319,6 +327,10 @@ pub enum WorkspaceEvent {
     PaneSelected {
         pane: PaneId,
     },
+    PaneAttached {
+        target: PaneId,
+        pane: PaneId,
+    },
     PaneRenamed {
         pane: PaneId,
     },
@@ -366,6 +378,7 @@ pub struct WorkspaceState {
     id: WorkspaceId,
     name: String,
     panes: BTreeMap<PaneId, PaneDefinition>,
+    detached_panes: BTreeMap<PaneId, PaneDefinition>,
     layout: LayoutNode,
     selected: PaneId,
     applied_requests: BTreeMap<CommandRequestId, WorkspaceRequest>,
@@ -396,6 +409,7 @@ impl WorkspaceState {
             id,
             name,
             panes,
+            detached_panes: BTreeMap::new(),
             layout,
             selected,
             applied_requests: BTreeMap::new(),
@@ -433,11 +447,21 @@ impl WorkspaceState {
         self.panes.get(id)
     }
 
+    #[must_use]
+    pub fn detached_pane(&self, id: &PaneId) -> Option<&PaneDefinition> {
+        self.detached_panes.get(id)
+    }
+
     #[must_use]
     pub fn pane_count(&self) -> usize {
         self.panes.len()
     }
 
+    #[must_use]
+    pub fn detached_pane_count(&self) -> usize {
+        self.detached_panes.len()
+    }
+
     /// Applies one idempotent command after checking its required capability.
     ///
     /// # Errors
@@ -511,7 +535,7 @@ impl WorkspaceState {
                 if !self.layout.contains(&target) {
                     return Err(WorkspaceError::PaneNotFound(target));
                 }
-                if self.panes.contains_key(&pane.id) {
+                if self.panes.contains_key(&pane.id) || self.detached_panes.contains_key(&pane.id) {
                     return Err(WorkspaceError::PaneAlreadyExists(pane.id));
                 }
                 pane.validate()?;
@@ -527,6 +551,31 @@ impl WorkspaceState {
                     pane: pane_id,
                 })
             }
+            WorkspaceCommand::AttachPane {
+                target,
+                pane,
+                axis,
+                ratio,
+                placement,
+            } => {
+                if !self.layout.contains(&target) {
+                    return Err(WorkspaceError::PaneNotFound(target));
+                }
+                if self.panes.contains_key(&pane) {
+                    return Err(WorkspaceError::PaneAlreadyExists(pane));
+                }
+                let definition = self
+                    .detached_panes
+                    .remove(&pane)
+                    .ok_or_else(|| WorkspaceError::PaneNotFound(pane.clone()))?;
+                let did_split = self
+                    .layout
+                    .split(&target, pane.clone(), axis, ratio, placement);
+                debug_assert!(did_split, "target presence was checked");
+                self.panes.insert(pane.clone(), definition);
+                self.selected = pane.clone();
+                Ok(WorkspaceEvent::PaneAttached { target, pane })
+            }
             WorkspaceCommand::SelectPane { pane } => {
                 if !self.panes.contains_key(&pane) {
                     return Err(WorkspaceError::PaneNotFound(pane));
@@ -546,6 +595,11 @@ impl WorkspaceState {
                 Ok(WorkspaceEvent::PaneRenamed { pane })
             }
             WorkspaceCommand::ClosePane { pane, disposition } => {
+                if disposition == PaneCloseDisposition::Terminate
+                    && self.detached_panes.remove(&pane).is_some()
+                {
+                    return Ok(WorkspaceEvent::PaneClosed { pane, disposition });
+                }
                 if self.panes.len() == 1 && self.panes.contains_key(&pane) {
                     return Err(WorkspaceError::CannotCloseLastPane);
                 }
@@ -557,7 +611,10 @@ impl WorkspaceState {
                     .clone()
                     .remove(&pane)
                     .ok_or(WorkspaceError::CannotCloseLastPane)?;
-                self.panes.remove(&pane);
+                let definition = self.panes.remove(&pane).expect("pane presence was checked");
+                if disposition == PaneCloseDisposition::Detach {
+                    self.detached_panes.insert(pane.clone(), definition);
+                }
                 self.layout = layout;
                 if self.selected == pane {
                     self.selected = self.layout.first_pane().clone();
@@ -580,6 +637,11 @@ impl fmt::Debug for WorkspaceEvent {
                 .debug_struct("PaneSelected")
                 .field("pane", pane)
                 .finish(),
+            Self::PaneAttached { target, pane } => formatter
+                .debug_struct("PaneAttached")
+                .field("target", target)
+                .field("pane", pane)
+                .finish(),
             Self::PaneRenamed { pane } => formatter
                 .debug_struct("PaneRenamed")
                 .field("pane", pane)
@@ -828,6 +890,8 @@ mod tests {
             .unwrap();
         assert_eq!(state.selected().as_str(), "root");
         assert_eq!(state.pane_count(), 1);
+        assert_eq!(state.detached_pane_count(), 1);
+        assert!(state.detached_pane(&id(PaneId::new, "second")).is_some());
         assert!(matches!(
             state.apply(
                 request(
@@ -843,6 +907,110 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn detached_pane_can_be_reattached_without_a_new_launch() {
+        let mut state = workspace();
+        state
+            .apply(
+                request(
+                    "split",
+                    WorkspaceCommand::SplitPane {
+                        target: id(PaneId::new, "root"),
+                        pane: pane("agent", true),
+                        axis: SplitAxis::Horizontal,
+                        ratio: SplitRatio::default(),
+                        placement: PanePlacement::After,
+                    },
+                ),
+                WorkspaceCapability::Execute,
+            )
+            .unwrap();
+        state
+            .apply(
+                request(
+                    "detach",
+                    WorkspaceCommand::ClosePane {
+                        pane: id(PaneId::new, "agent"),
+                        disposition: PaneCloseDisposition::Detach,
+                    },
+                ),
+                WorkspaceCapability::Configure,
+            )
+            .unwrap();
+
+        assert_eq!(state.pane_count(), 1);
+        assert_eq!(state.detached_pane_count(), 1);
+        state
+            .apply(
+                request(
+                    "attach",
+                    WorkspaceCommand::AttachPane {
+                        target: id(PaneId::new, "root"),
+                        pane: id(PaneId::new, "agent"),
+                        axis: SplitAxis::Horizontal,
+                        ratio: SplitRatio::default(),
+                        placement: PanePlacement::After,
+                    },
+                ),
+                WorkspaceCapability::Configure,
+            )
+            .unwrap();
+
+        assert_eq!(state.pane_count(), 2);
+        assert_eq!(state.detached_pane_count(), 0);
+        assert_eq!(state.selected().as_str(), "agent");
+        assert!(state.pane(&id(PaneId::new, "agent")).is_some());
+    }
+
+    #[test]
+    fn terminating_a_detached_pane_requires_execute_and_removes_it() {
+        let mut state = workspace();
+        state
+            .apply(
+                request(
+                    "split",
+                    WorkspaceCommand::SplitPane {
+                        target: id(PaneId::new, "root"),
+                        pane: pane("agent", false),
+                        axis: SplitAxis::Horizontal,
+                        ratio: SplitRatio::default(),
+                        placement: PanePlacement::After,
+                    },
+                ),
+                WorkspaceCapability::Configure,
+            )
+            .unwrap();
+        state
+            .apply(
+                request(
+                    "detach",
+                    WorkspaceCommand::ClosePane {
+                        pane: id(PaneId::new, "agent"),
+                        disposition: PaneCloseDisposition::Detach,
+                    },
+                ),
+                WorkspaceCapability::Configure,
+            )
+            .unwrap();
+
+        let terminate = request(
+            "terminate",
+            WorkspaceCommand::ClosePane {
+                pane: id(PaneId::new, "agent"),
+                disposition: PaneCloseDisposition::Terminate,
+            },
+        );
+        assert!(matches!(
+            state.apply(terminate.clone(), WorkspaceCapability::Configure),
+            Err(WorkspaceError::CapabilityDenied { .. })
+        ));
+        state
+            .apply(terminate, WorkspaceCapability::Execute)
+            .unwrap();
+        assert_eq!(state.detached_pane_count(), 0);
+        assert_eq!(state.pane_count(), 1);
+    }
+
     #[test]
     fn rejects_invalid_identifiers_ratios_and_launch_surfaces() {
         assert!(PaneId::new(" ").is_err());

docs/ARCHITECTURE.md

@@ -89,6 +89,12 @@ approval boundary. The selected panel alone owns keyboard input. Six surfaces
 continue updating so performance comparisons remain meaningful, with a sliding
 visible window keeping the selected pane onscreen. See decisions 0008 and 0009.
 
+Panel attachment is independent from session lifetime. Detaching removes a pane
+from the layout tree but retains its definition in the workspace's detached
+registry and leaves its runtime-owned process alive. Reattaching restores that
+identity without a launch. Terminating an attached or detached session is a
+separate Execute-capability operation. See decision 0010.
+
 We should evaluate, not blindly copy, WezTerm, Zellij, RMUX, tmux, and cmux. The
 first spike must compare a reusable terminal crate with a small first-party layer.
 Remaining correctness cases include OSC 8 links, Kitty

docs/PRODUCT_SPEC.md

@@ -79,6 +79,13 @@ capability each choice would need. Suggestions are inert data. Choosing one may
 prepare a typed workspace plan, but any command, file mutation, credential use,
 or external message still passes through the normal approval and command plane.
 
+Removing a panel detaches it from the layout; it does not terminate the pane's
+PTY, agent, browser, or remote session. Detached sessions remain visible in a
+workspace tray and can be reattached without relaunching. Termination is a
+separate, explicitly named Execute-capability action with confirmation. The add
+control creates the default panel beside the selection, with a menu for choosing
+a surface or reattaching a running session. At least one panel remains attached.
+
 ## Lumbridge Harness
 
 Lumbridge Harness is an optional orchestrator distributed as a separately

docs/RESEARCH.md

@@ -11,7 +11,8 @@ They are shallow snapshots for study, not dependencies or vendored source.
 ## Multiplexers and terminal engines
 
 - `manaflow-ai/cmux`: the current cmux product; native workspace/pane model,
-  automation socket, embedded browser, agent hooks, and terminal UX.
+  automation socket, embedded browser, agent hooks, and terminal UX. The
+  captured tree is GPL-3.0-or-later, so it is behavior/UX study only.
 - `Helvesec/rmux`: Rust multiplexer engine, daemon, typed SDKs, tmux surface.
 - `tmux/tmux`: the durable client/server and command model to remain compatible
   with where useful.

docs/TESTING.md

@@ -104,7 +104,8 @@ who already have the final cargo-watch release installed.
 ## Implemented vertical-slice coverage
 
 - The framework-neutral six-surface model has deterministic tests for focus,
-  direct selection, needs-input transitions, command-palette lifecycle,
+  direct selection, reversible attach/detach, last-panel protection, detached
+  background updates, needs-input transitions, command-palette lifecycle,
   identical GPUI/Floem action replay, bounded output, and workload counters.
 - `lumbridge-pty` runs synthetic `/bin/sh` tests for raw output, non-zero exits,
   input, resize, one-chunk backpressure, hung-process termination, invalid
@@ -117,7 +118,8 @@ who already have the final cargo-watch release installed.
   alternate screen, bracketed paste, protocol replies, sanitized title and
   blocked OSC 52 behavior, key encoding, application-cursor mode, and resize.
 - `lumbridge-core` tests capability-gated split/select/rename/close commands,
-  idempotent request IDs, close-tree promotion, and atomic agentic setup plans.
+  idempotent request IDs, close-tree promotion, detached-pane reattachment,
+  Execute-gated detached termination, and atomic agentic setup plans.
 - The GPUI slice tests its key-event adapter, responsive one/three/five-panel
   geometry, selected-panel windowing, per-panel 20/60/20 PTY sizing, xterm
   256-color conversion, styled-run coalescing, cursor-run boundaries, and

docs/UI_SPIKE_SCORECARD.md

@@ -25,8 +25,11 @@ The current programs establish dependency, build, launch, and interaction
 baselines. GPUI now routes a real PTY through a VT engine and keyboard encoder,
 paints coalesced styled cell runs and cursor shapes, and derives PTY rows/columns
 from the 60% terminal region inside each responsive one/three/five-panel layout
-when the window changes. Retained-history page/top/bottom navigation is wired. Text selection,
-mouse modes, a native Markdown editor, and one isolated browser child remain.
+when the window or attached panel count changes. Retained-history
+page/top/bottom navigation is wired. Panel detach/reattach keeps background
+surface updates alive and exposes detached sessions in the sidebar. Text
+selection, mouse modes, a native Markdown editor, and one isolated browser child
+remain.
 
 ## Ubuntu baseline — metal, 2026-08-31
 

docs/UX_VERTICAL_SLICE.md

@@ -19,6 +19,11 @@ Remaining visible gaps are terminal text selection and mouse modes, real
 decision-shelf actions, usage-detail provenance, and end-to-end platform
 accessibility/IME.
 
+Panel lifecycle is now visible in the slice: every panel has a Detach action,
+the workspace bar has Add panel, and the sidebar exposes Detached sessions for
+reattachment. The wording is deliberate because panel removal keeps its session
+alive; termination is not exposed as a visually equivalent close action.
+
 Screenshots for the audit are stored outside Git under
 `~/shots/2026-08/lumbridge-ui-audit/`. Accessibility and IME correctness cannot
 be established from screenshots and remain explicit runtime gates.
@@ -82,6 +87,8 @@ surface rather than a hosted Lumbridge control plane.
   leaving ordinary terminal arrows and text available to the PTY.
 - `Alt+1` through `Alt+6`: focus a pane directly while leaving terminal digits
   available to the PTY.
+- `Alt+Shift+N`: add or reattach the next panel in the deterministic slice.
+- `Alt+Shift+W`: detach the selected panel while its session keeps running.
 - `Cmd+K` on macOS or `Ctrl+K` on Linux: open the command palette.
 - `Shift+PageUp/PageDown`: move the selected live terminal through retained
   history; `Shift+Home/End` jumps to the history top/live bottom.

docs/decisions/0006-workspace-command-plane.md

@@ -14,6 +14,11 @@ process requires Execute; layout-only changes require Configure. Launch intents
 refer to a user shell or an approved harness profile and working directory, not
 arbitrary executable arguments or secret values.
 
+Detaching and reattaching an existing pane are layout-only Configure operations.
+Detached pane definitions remain outside the layout tree so reattachment does
+not imply a new launch. Terminating either an attached or detached session
+remains an Execute operation. See decision 0010.
+
 This reducer is not an authorization bypass. The future IPC layer authenticates
 the caller and supplies its granted capability; the reducer rechecks it before
 mutation. Durable audit events record request and object identities, never

docs/decisions/0010-panel-lifecycle-detach-before-terminate.md

@@ -0,0 +1,37 @@
+# 0010: Removing a panel detaches its session before termination
+
+Status: accepted for implementation.
+
+A workspace panel is a view attachment, not the lifetime owner of a PTY, agent,
+browser, or remote session. The normal remove action is therefore **Detach
+panel**. It removes the panel from the visible layout, keeps its stable pane and
+session identities, preserves output and decision state, and exposes the item in
+a Detached sessions tray. Reattaching it is a Configure-capability operation and
+must not launch a second process.
+
+Termination is a separate Execute-capability operation. It is presented as
+**Terminate session**, never as an ambiguous close icon, and will require a
+confirmation that names the process, target machine, and any dirty or waiting
+state. Detaching the selected panel chooses its next attached sibling, falling
+back to the previous sibling. A workspace retains at least one attached panel.
+
+The add affordance has two product paths: quick activation creates the user's
+default panel beside the selection, while its menu offers Terminal, Browser,
+Markdown, Review, and Reattach running session. The deterministic UI spike uses
+six fixed fixtures, starts with five attached, and reattaches the first detached
+fixture when Add panel is activated. That fixture limit is test scaffolding, not
+a product limit.
+
+The typed command plane retains detached pane definitions separately from the
+layout tree. `ClosePane(Detach)` moves a definition into that registry,
+`AttachPane` restores it without a launch intent, and `ClosePane(Terminate)`
+requires Execute authority whether the pane is attached or detached. The future
+runtime registry remains the process owner throughout these layout transitions.
+
+This interaction vocabulary was informed by behavior study of cmux at
+`fc36aea87e3b4152637597443d7aa7c22e4ec9f8` and Orca at
+`02a7742406a5a84fb372d6255d5a4367421990bd`. cmux is GPL-3.0-or-later and was
+used only to study workspace/split shortcuts and close-versus-detached process
+behavior. Orca is MIT-licensed and was used to study worktree-first navigation,
+durable terminal state, and mixed surface density. No implementation code or
+visual asset was copied from either project.

spikes/floem-shell/src/main.rs

@@ -346,7 +346,7 @@ fn handle_key(
 }
 
 fn app_view() -> impl IntoView {
-    let model = RwSignal::new(ShellModel::default());
+    let model = RwSignal::new(ShellModel::with_all_panels_attached());
     let query = RwSignal::new(String::new());
     let timer_pulse = RwSignal::new(());
 

spikes/gpui-shell/src/main.rs

@@ -10,8 +10,8 @@ use lumbridge_runtime::{
     RuntimeCommand, RuntimeEvent, TerminalSize,
 };
 use lumbridge_spike_model::{
-    ActionOutcome, FOOTER_RIGHT, FocusDirection, OutputSource, PaneId, PaneState, ShellAction,
-    ShellModel, SurfaceKind, WORKSPACES,
+    ActionOutcome, FOOTER_RIGHT, FocusDirection, INITIAL_ATTACHED_PANEL_COUNT, OutputSource,
+    PaneId, PaneState, ShellAction, ShellModel, SurfaceKind, WORKSPACES,
 };
 use lumbridge_terminal::{
     KeyModifiers, TerminalCellStyle, TerminalColor, TerminalCursorShape, TerminalDimensions,
@@ -62,6 +62,8 @@ actions!(
         SelectPane4,
         SelectPane5,
         SelectPane6,
+        AddPanel,
+        DetachSelectedPanel,
         TerminalTaller,
         TerminalShorter,
         TerminalWider,
@@ -272,14 +274,18 @@ fn dim_color(color: u32) -> u32 {
     (dim((color >> 16) & 0xff) << 16) | (dim((color >> 8) & 0xff) << 8) | dim(color & 0xff)
 }
 
-fn terminal_dimensions_for_window(window_size: Size<Pixels>) -> TerminalDimensions {
+fn terminal_dimensions_for_window(
+    window_size: Size<Pixels>,
+    attached_panel_count: usize,
+) -> TerminalDimensions {
     let width = f32::from(window_size.width);
     let height = f32::from(window_size.height);
-    let panel_count = visible_panel_count(window_size);
+    let panel_count = visible_panel_count(window_size)
+        .min(attached_panel_count)
+        .max(1);
     let workspace_height =
         (height - APP_HEADER_HEIGHT - TAB_BAR_HEIGHT - APP_FOOTER_HEIGHT).max(0.0);
-    let terminal_height =
-        (workspace_height * 0.60 - TERMINAL_CONTENT_VERTICAL_INSET).max(0.0);
+    let terminal_height = (workspace_height * 0.60 - TERMINAL_CONTENT_VERTICAL_INSET).max(0.0);
     let workspace_width = (width - SIDEBAR_WIDTH).max(0.0);
     let panel_gaps = WORK_PANEL_GAP * panel_count.saturating_sub(1) as f32;
     let panel_width = ((workspace_width - panel_gaps).max(0.0) / panel_count as f32).max(0.0);
@@ -401,7 +407,8 @@ impl LumbridgeShell {
         })
         .detach();
 
-        let terminal_dimensions = terminal_dimensions_for_window(window.bounds().size);
+        let terminal_dimensions =
+            terminal_dimensions_for_window(window.bounds().size, INITIAL_ATTACHED_PANEL_COUNT);
         let terminal = TerminalEngine::new(TerminalEngineOptions {
             dimensions: terminal_dimensions,
             ..TerminalEngineOptions::default()
@@ -413,7 +420,10 @@ impl LumbridgeShell {
         };
 
         cx.observe_window_bounds(window, |shell, window, cx| {
-            let dimensions = terminal_dimensions_for_window(window.bounds().size);
+            let dimensions = terminal_dimensions_for_window(
+                window.bounds().size,
+                shell.model.attached_panel_count(),
+            );
             shell.resize_terminal(dimensions.rows(), dimensions.columns(), cx);
         })
         .detach();
@@ -559,6 +569,50 @@ impl LumbridgeShell {
         cx.notify();
     }
 
+    fn resize_terminal_for_workspace(&mut self, window: &Window, cx: &mut Context<Self>) {
+        let dimensions =
+            terminal_dimensions_for_window(window.bounds().size, self.model.attached_panel_count());
+        self.resize_terminal(dimensions.rows(), dimensions.columns(), cx);
+    }
+
+    fn attach_panel(&mut self, pane: PaneId, window: &mut Window, cx: &mut Context<Self>) {
+        let outcome = self.dispatch(ShellAction::AttachPanel(pane));
+        if outcome.changed {
+            self.resize_terminal_for_workspace(window, cx);
+        }
+        window.focus(&self.root_focus);
+        cx.notify();
+    }
+
+    fn add_panel(&mut self, window: &mut Window, cx: &mut Context<Self>) {
+        let Some(pane) = self.model.next_detached_panel() else {
+            return;
+        };
+        self.attach_panel(pane, window, cx);
+    }
+
+    fn detach_panel(&mut self, pane: PaneId, window: &mut Window, cx: &mut Context<Self>) {
+        let outcome = self.dispatch(ShellAction::DetachPanel(pane));
+        if outcome.changed {
+            self.resize_terminal_for_workspace(window, cx);
+        }
+        window.focus(&self.root_focus);
+        cx.notify();
+    }
+
+    fn add_panel_action(&mut self, _: &AddPanel, window: &mut Window, cx: &mut Context<Self>) {
+        self.add_panel(window, cx);
+    }
+
+    fn detach_selected_panel(
+        &mut self,
+        _: &DetachSelectedPanel,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        self.detach_panel(self.model.selected_pane(), window, cx);
+    }
+
     fn move_focus(
         &mut self,
         direction: FocusDirection,
@@ -641,10 +695,9 @@ impl LumbridgeShell {
                 return;
             }
             let modifiers = key_modifiers(event);
-            if let Some(scroll) = terminal_scroll_from_parts(
-                event.keystroke.key.as_str(),
-                modifiers,
-            ) {
+            if let Some(scroll) =
+                terminal_scroll_from_parts(event.keystroke.key.as_str(), modifiers)
+            {
                 self.scroll_terminal(scroll, cx);
                 return;
             }
@@ -768,7 +821,14 @@ impl LumbridgeShell {
             .into_any_element()
     }
 
-    fn pane_context(&self, pane: &PaneState, selected: bool) -> gpui::AnyElement {
+    fn pane_context(
+        &self,
+        pane: &PaneState,
+        selected: bool,
+        cx: &mut Context<Self>,
+    ) -> gpui::AnyElement {
+        let pane_id = pane.id();
+        let can_detach = self.model.attached_panel_count() > 1;
         let external = pane.output_source() == OutputSource::External;
         let status = if external {
             self.runtime_status.badge()
@@ -859,7 +919,13 @@ impl LumbridgeShell {
                     )
                     .child(
                         div()
+                            .flex()
+                            .flex_col()
+                            .items_end()
                             .flex_none()
+                            .gap_1()
+                            .child(
+                                div()
                                     .text_xs()
                                     .text_color(rgb(if pane.needs_input() {
                                         ATTENTION
@@ -869,6 +935,28 @@ impl LumbridgeShell {
                                         MUTED
                                     }))
                                     .child(surface_status),
+                            )
+                            .child(
+                                div()
+                                    .id(("detach-panel", pane_id.index()))
+                                    .cursor_pointer()
+                                    .px_2()
+                                    .py_1()
+                                    .rounded(px(4.0))
+                                    .border_1()
+                                    .border_color(rgb(BORDER))
+                                    .text_xs()
+                                    .text_color(rgb(if can_detach { MUTED } else { BORDER }))
+                                    .child(if can_detach {
+                                        "− DETACH"
+                                    } else {
+                                        "LAST PANEL"
+                                    })
+                                    .on_click(cx.listener(move |shell, _, window, cx| {
+                                        cx.stop_propagation();
+                                        shell.detach_panel(pane_id, window, cx);
+                                    })),
+                            ),
                     ),
             )
             .child(
@@ -990,18 +1078,10 @@ impl LumbridgeShell {
                     .items_center()
                     .justify_between()
                     .text_xs()
+                    .child(div().text_color(rgb(MUTED)).child("DECISION SHELF"))
                     .child(
                         div()
-                            .text_color(rgb(MUTED))
-                            .child("DECISION SHELF"),
-                    )
-                    .child(
-                        div()
-                            .text_color(rgb(if pane.needs_input() {
-                                ATTENTION
-                            } else {
-                                MUTED
-                            }))
+                            .text_color(rgb(if pane.needs_input() { ATTENTION } else { MUTED }))
                             .child(if pane.needs_input() {
                                 "REVIEW REQUIRED"
                             } else {
@@ -1009,16 +1089,9 @@ impl LumbridgeShell {
                             }),
                     ),
             )
-            .child(
-                div()
-                    .flex()
-                    .flex_col()
-                    .gap_1()
-                    .mt_2()
-                    .children(choices.map(|(label, detail, attention)| {
-                        choice(label, detail, attention)
-                    })),
-            )
+            .child(div().flex().flex_col().gap_1().mt_2().children(
+                choices.map(|(label, detail, attention)| choice(label, detail, attention)),
+            ))
             .into_any_element()
     }
 
@@ -1048,7 +1121,7 @@ impl LumbridgeShell {
                     .flex_none()
                     .min_h_0()
                     .overflow_hidden()
-                    .child(self.pane_context(pane, selected)),
+                    .child(self.pane_context(pane, selected, cx)),
             )
             .child(
                 div()
@@ -1128,7 +1201,26 @@ impl LumbridgeShell {
                                     .px_3()
                                     .py_2()
                                     .text_color(rgb(MUTED))
-                                    .child("Open attention request"),
+                                    .child("Add workspace panel")
+                                    .child(
+                                        div()
+                                            .mt_1()
+                                            .text_xs()
+                                            .child("Alt+Shift+N · reattach if detached"),
+                                    ),
+                            )
+                            .child(
+                                div()
+                                    .px_3()
+                                    .py_2()
+                                    .text_color(rgb(MUTED))
+                                    .child("Detach selected panel")
+                                    .child(
+                                        div()
+                                            .mt_1()
+                                            .text_xs()
+                                            .child("Alt+Shift+W · session keeps running"),
+                                    ),
                             )
                             .child(
                                 div()
@@ -1158,7 +1250,17 @@ impl LumbridgeShell {
 impl Render for LumbridgeShell {
     fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let attention = self.model.pane(PaneId::ClaudeUi);
-        let panel_count = visible_panel_count(window.bounds().size);
+        let panel_capacity = visible_panel_count(window.bounds().size);
+        let attached_panes = self.model.attached_pane_ids();
+        let attached_count = attached_panes.len();
+        let detached_entries = self
+            .model
+            .detached_pane_ids()
+            .into_iter()
+            .map(|pane| (pane, self.model.pane(pane).fixture().title))
+            .collect::<Vec<_>>();
+        let detached_count = detached_entries.len();
+        let visible_count = panel_capacity.min(attached_count).max(1);
         let sidebar = div()
             .flex()
             .flex_col()
@@ -1263,6 +1365,33 @@ impl Render for LumbridgeShell {
                     .text_color(rgb(MUTED))
                     .child("spark-1 · sleeping"),
             )
+            .child(
+                div()
+                    .mt_3()
+                    .px_4()
+                    .py_2()
+                    .text_xs()
+                    .text_color(rgb(MUTED))
+                    .child(format!("DETACHED SESSIONS · {detached_count}")),
+            )
+            .children(detached_entries.into_iter().map(|(pane, title)| {
+                div()
+                    .id(("detached-session", pane.index()))
+                    .cursor_pointer()
+                    .mx_2()
+                    .mb_1()
+                    .px_3()
+                    .py_2()
+                    .rounded(px(4.0))
+                    .border_1()
+                    .border_color(rgb(BORDER_QUIET))
+                    .text_xs()
+                    .text_color(rgb(MUTED))
+                    .child(format!("↪ {title}"))
+                    .on_click(cx.listener(move |shell, _, window, cx| {
+                        shell.attach_panel(pane, window, cx);
+                    }))
+            }))
             .child(div().flex_1())
             .child(
                 div()
@@ -1317,33 +1446,44 @@ impl Render for LumbridgeShell {
                     .child("Review"),
             )
             .child(div().flex_1())
+            .child(div().px_3().text_xs().text_color(rgb(MUTED)).child(format!(
+                "{visible_count} shown · {attached_count} attached · {detached_count} detached"
+            )))
             .child(
                 div()
+                    .id("add-workspace-panel")
+                    .cursor_pointer()
+                    .ml_2()
                     .px_3()
+                    .py_1()
+                    .rounded(px(4.0))
+                    .border_1()
+                    .border_color(rgb(if detached_count > 0 { ACCENT } else { BORDER }))
                     .text_xs()
-                    .text_color(rgb(MUTED))
-                    .child(format!(
-                        "{panel_count} visible panels · 1 interactive VT · 1 waiting"
-                    )),
+                    .text_color(rgb(if detached_count > 0 { ACCENT } else { MUTED }))
+                    .child(if detached_count > 0 {
+                        "+ ADD PANEL"
+                    } else {
+                        "ALL PANELS ATTACHED"
+                    })
+                    .on_click(cx.listener(|shell, _, window, cx| {
+                        shell.add_panel(window, cx);
+                    })),
             );
 
-        let panel_range = visible_panel_range(
-            PaneId::ALL.len(),
-            self.model.selected_pane().index(),
-            panel_count,
-        );
+        let selected_position = attached_panes
+            .iter()
+            .position(|pane| *pane == self.model.selected_pane())
+            .expect("the selected pane must remain attached");
+        let panel_range = visible_panel_range(attached_count, selected_position, visible_count);
         let workspace_panels = panel_range
             .map(|index| {
-                let pane = self.model.pane(PaneId::ALL[index]);
+                let pane = self.model.pane(attached_panes[index]);
                 div()
                     .flex_1()
                     .min_w_0()
                     .min_h_0()
-                    .child(self.workspace_panel(
-                        pane,
-                        pane.id() == self.model.selected_pane(),
-                        cx,
-                    ))
+                    .child(self.workspace_panel(pane, pane.id() == self.model.selected_pane(), cx))
             })
             .collect::<Vec<_>>();
         let workspace_row = div()
@@ -1380,6 +1520,8 @@ impl Render for LumbridgeShell {
             .on_action(cx.listener(Self::focus_down))
             .on_action(cx.listener(Self::open_palette))
             .on_action(cx.listener(Self::close_palette))
+            .on_action(cx.listener(Self::add_panel_action))
+            .on_action(cx.listener(Self::detach_selected_panel))
             .on_action(cx.listener(Self::terminal_taller))
             .on_action(cx.listener(Self::terminal_shorter))
             .on_action(cx.listener(Self::terminal_wider))
@@ -1570,6 +1712,8 @@ fn main() {
             KeyBinding::new("alt-4", SelectPane4, Some("LumbridgeShell")),
             KeyBinding::new("alt-5", SelectPane5, Some("LumbridgeShell")),
             KeyBinding::new("alt-6", SelectPane6, Some("LumbridgeShell")),
+            KeyBinding::new("alt-shift-n", AddPanel, Some("LumbridgeShell")),
+            KeyBinding::new("alt-shift-w", DetachSelectedPanel, Some("LumbridgeShell")),
             KeyBinding::new("alt-shift-up", TerminalTaller, Some("LumbridgeShell")),
             KeyBinding::new("alt-shift-down", TerminalShorter, Some("LumbridgeShell")),
             KeyBinding::new("alt-shift-right", TerminalWider, Some("LumbridgeShell")),
@@ -1626,15 +1770,19 @@ mod tests {
 
     #[test]
     fn terminal_geometry_tracks_middle_sixty_percent_per_panel() {
-        let dimensions = terminal_dimensions_for_window(size(px(1500.0), px(960.0)));
+        let dimensions = terminal_dimensions_for_window(size(px(1500.0), px(960.0)), 5);
         assert_eq!(dimensions.rows(), 26);
         assert_eq!(dimensions.columns(), 45);
 
         let ultrawide = size(px(3440.0), px(1440.0));
         assert_eq!(visible_panel_count(ultrawide), 5);
-        let dimensions = terminal_dimensions_for_window(ultrawide);
+        let dimensions = terminal_dimensions_for_window(ultrawide, 5);
         assert_eq!(dimensions.rows(), 42);
         assert_eq!(dimensions.columns(), 71);
+
+        let two_panels = terminal_dimensions_for_window(ultrawide, 2);
+        assert_eq!(two_panels.rows(), 42);
+        assert_eq!(two_panels.columns(), 185);
     }
 
     #[test]

spikes/ui-shell-model/src/lib.rs

@@ -7,6 +7,7 @@ use std::fmt;
 
 pub const GRID_ROWS: usize = 2;
 pub const GRID_COLUMNS: usize = 3;
+pub const INITIAL_ATTACHED_PANEL_COUNT: usize = 5;
 pub const DEFAULT_TERMINAL_LINE_LIMIT: usize = 64;
 
 /// Stable identity used in traces and accessibility identifiers.
@@ -282,6 +283,8 @@ pub enum FocusDirection {
 pub enum ShellAction {
     MoveFocus(FocusDirection),
     SelectPane(PaneId),
+    AttachPanel(PaneId),
+    DetachPanel(PaneId),
     SetNeedsInput { pane: PaneId, needs_input: bool },
     OpenCommandPalette,
     CloseCommandPalette,
@@ -301,6 +304,9 @@ pub struct MeasurementCounters {
     pub focus_moves: u64,
     pub blocked_focus_moves: u64,
     pub direct_selections: u64,
+    pub panel_attaches: u64,
+    pub panel_detaches: u64,
+    pub blocked_panel_detaches: u64,
     pub needs_input_changes: u64,
     pub palette_changes: u64,
     pub synthetic_ticks: u64,
@@ -344,6 +350,7 @@ impl std::error::Error for InvalidExternalOutputPane {}
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct ShellModel {
     panes: [PaneState; GRID_ROWS * GRID_COLUMNS],
+    attached_panels: [bool; GRID_ROWS * GRID_COLUMNS],
     selected_pane: PaneId,
     command_palette: CommandPaletteState,
     terminal_line_limit: usize,
@@ -381,6 +388,7 @@ impl ShellModel {
         });
         Ok(Self {
             panes,
+            attached_panels: std::array::from_fn(|index| index < INITIAL_ATTACHED_PANEL_COUNT),
             selected_pane: PaneId::CodexRuntime,
             command_palette: CommandPaletteState::default(),
             terminal_line_limit: limit,
@@ -406,6 +414,14 @@ impl ShellModel {
         Ok(model)
     }
 
+    /// Creates the legacy six-card comparison state used by the Floem spike.
+    #[must_use]
+    pub fn with_all_panels_attached() -> Self {
+        let mut model = Self::default();
+        model.attached_panels.fill(true);
+        model
+    }
+
     #[must_use]
     pub fn panes(&self) -> &[PaneState; GRID_ROWS * GRID_COLUMNS] {
         &self.panes
@@ -415,6 +431,41 @@ impl ShellModel {
         &self.panes[id.index()]
     }
     #[must_use]
+    pub fn attached_pane_ids(&self) -> Vec<PaneId> {
+        PaneId::ALL
+            .into_iter()
+            .filter(|pane| self.is_panel_attached(*pane))
+            .collect()
+    }
+    #[must_use]
+    pub fn detached_pane_ids(&self) -> Vec<PaneId> {
+        PaneId::ALL
+            .into_iter()
+            .filter(|pane| !self.is_panel_attached(*pane))
+            .collect()
+    }
+    #[must_use]
+    pub const fn is_panel_attached(&self, pane: PaneId) -> bool {
+        self.attached_panels[pane.index()]
+    }
+    #[must_use]
+    pub fn attached_panel_count(&self) -> usize {
+        self.attached_panels
+            .iter()
+            .filter(|attached| **attached)
+            .count()
+    }
+    #[must_use]
+    pub fn detached_panel_count(&self) -> usize {
+        self.attached_panels.len() - self.attached_panel_count()
+    }
+    #[must_use]
+    pub fn next_detached_panel(&self) -> Option<PaneId> {
+        PaneId::ALL
+            .into_iter()
+            .find(|pane| !self.is_panel_attached(*pane))
+    }
+    #[must_use]
     pub const fn selected_pane(&self) -> PaneId {
         self.selected_pane
     }
@@ -445,7 +496,9 @@ impl ShellModel {
         let mut appended = 0;
         match action {
             ShellAction::MoveFocus(direction) => {
-                if let Some(next) = focus_neighbor(self.selected_pane, direction) {
+                if let Some(next) =
+                    attached_focus_neighbor(&self.attached_panels, self.selected_pane, direction)
+                {
                     self.selected_pane = next;
                     self.counters.focus_moves += 1;
                     changed = true;
@@ -454,12 +507,41 @@ impl ShellModel {
                 }
             }
             ShellAction::SelectPane(pane) => {
-                if self.selected_pane != pane {
+                if self.is_panel_attached(pane) && self.selected_pane != pane {
                     self.selected_pane = pane;
                     self.counters.direct_selections += 1;
                     changed = true;
                 }
             }
+            ShellAction::AttachPanel(pane) => {
+                if !self.is_panel_attached(pane) {
+                    self.attached_panels[pane.index()] = true;
+                    self.selected_pane = pane;
+                    self.counters.panel_attaches += 1;
+                    changed = true;
+                }
+            }
+            ShellAction::DetachPanel(pane) => {
+                if self.is_panel_attached(pane) {
+                    if self.attached_panel_count() == 1 {
+                        self.counters.blocked_panel_detaches += 1;
+                    } else {
+                        let attached_before = self.attached_pane_ids();
+                        let detached_position = attached_before
+                            .iter()
+                            .position(|candidate| *candidate == pane)
+                            .expect("attached panel must appear in attached IDs");
+                        self.attached_panels[pane.index()] = false;
+                        if self.selected_pane == pane {
+                            let remaining = self.attached_pane_ids();
+                            self.selected_pane =
+                                remaining[detached_position.min(remaining.len().saturating_sub(1))];
+                        }
+                        self.counters.panel_detaches += 1;
+                        changed = true;
+                    }
+                }
+            }
             ShellAction::SetNeedsInput { pane, needs_input } => {
                 let pane = &mut self.panes[pane.index()];
                 if needs_input && !pane.needs_input() {
@@ -570,13 +652,9 @@ impl ShellModel {
 
 #[must_use]
 pub const fn focus_neighbor(pane: PaneId, direction: FocusDirection) -> Option<PaneId> {
-    let row = pane.row();
-    let column = pane.column();
     let next = match direction {
-        FocusDirection::Left if column > 0 => Some(pane.index() - 1),
-        FocusDirection::Right if column + 1 < GRID_COLUMNS => Some(pane.index() + 1),
-        FocusDirection::Up if row > 0 => Some(pane.index() - GRID_COLUMNS),
-        FocusDirection::Down if row + 1 < GRID_ROWS => Some(pane.index() + GRID_COLUMNS),
+        FocusDirection::Left if pane.index() > 0 => Some(pane.index() - 1),
+        FocusDirection::Right if pane.index() + 1 < PaneId::ALL.len() => Some(pane.index() + 1),
         _ => None,
     };
     match next {
@@ -585,6 +663,25 @@ pub const fn focus_neighbor(pane: PaneId, direction: FocusDirection) -> Option<P
     }
 }
 
+fn attached_focus_neighbor(
+    attached: &[bool; GRID_ROWS * GRID_COLUMNS],
+    pane: PaneId,
+    direction: FocusDirection,
+) -> Option<PaneId> {
+    match direction {
+        FocusDirection::Left => PaneId::ALL[..pane.index()]
+            .iter()
+            .rev()
+            .copied()
+            .find(|candidate| attached[candidate.index()]),
+        FocusDirection::Right => PaneId::ALL[pane.index() + 1..]
+            .iter()
+            .copied()
+            .find(|candidate| attached[candidate.index()]),
+        FocusDirection::Up | FocusDirection::Down => None,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::{
@@ -613,6 +710,8 @@ mod tests {
     fn initial_state_has_one_explicit_needs_input_pane() {
         let model = ShellModel::default();
         assert_eq!(model.selected_pane(), PaneId::CodexRuntime);
+        assert_eq!(model.attached_panel_count(), 5);
+        assert_eq!(model.detached_pane_ids(), vec![PaneId::RuntimeReview]);
         assert_eq!(model.revision(), 0);
         assert!(!model.command_palette().is_open());
         assert_eq!(
@@ -627,24 +726,35 @@ mod tests {
     }
 
     #[test]
-    fn neighbors_match_two_by_three_grid_without_wrapping() {
+    fn legacy_comparison_constructor_attaches_all_six_without_an_event() {
+        let model = ShellModel::with_all_panels_attached();
+        assert_eq!(model.attached_panel_count(), PaneId::ALL.len());
+        assert_eq!(model.revision(), 0);
+        assert_eq!(model.counters().events_dispatched, 0);
+    }
+
+    #[test]
+    fn neighbors_follow_the_horizontal_panel_row_without_wrapping() {
         assert_eq!(
             focus_neighbor(PaneId::CodexRuntime, FocusDirection::Right),
             Some(PaneId::ClaudeUi)
         );
         assert_eq!(
-            focus_neighbor(PaneId::ClaudeUi, FocusDirection::Down),
-            Some(PaneId::AcpPreview)
+            focus_neighbor(PaneId::ClaudeUi, FocusDirection::Left),
+            Some(PaneId::CodexRuntime)
         );
         assert_eq!(
-            focus_neighbor(PaneId::RuntimeReview, FocusDirection::Up),
-            Some(PaneId::PiDocs)
+            focus_neighbor(PaneId::AcpPreview, FocusDirection::Right),
+            Some(PaneId::RuntimeReview)
         );
         assert_eq!(
-            focus_neighbor(PaneId::Architecture, FocusDirection::Left),
+            focus_neighbor(PaneId::CodexRuntime, FocusDirection::Left),
+            None
+        );
+        assert_eq!(
+            focus_neighbor(PaneId::RuntimeReview, FocusDirection::Right),
             None
         );
-        assert_eq!(focus_neighbor(PaneId::PiDocs, FocusDirection::Right), None);
         assert_eq!(
             focus_neighbor(PaneId::CodexRuntime, FocusDirection::Up),
             None
@@ -661,12 +771,79 @@ mod tests {
         assert!(moved.changed);
         assert_eq!(moved.selected_pane, PaneId::ClaudeUi);
         assert_eq!(moved.revision, 1);
-        model.dispatch(ShellAction::MoveFocus(FocusDirection::Down));
-        assert_eq!(model.selected_pane(), PaneId::AcpPreview);
+        model.dispatch(ShellAction::MoveFocus(FocusDirection::Right));
+        assert_eq!(model.selected_pane(), PaneId::PiDocs);
         assert_eq!(model.counters().focus_moves, 2);
         assert_eq!(model.counters().blocked_focus_moves, 1);
     }
 
+    #[test]
+    fn detach_is_reversible_and_keeps_the_surface_running() {
+        let mut model = ShellModel::default();
+        model.dispatch(ShellAction::SyntheticStreamTick);
+        let sequence_before = model.pane(PaneId::ClaudeUi).synthetic_line_sequence();
+
+        let detached = model.dispatch(ShellAction::DetachPanel(PaneId::ClaudeUi));
+        assert!(detached.changed);
+        assert!(!model.is_panel_attached(PaneId::ClaudeUi));
+        assert_eq!(model.attached_panel_count(), 4);
+
+        model.dispatch(ShellAction::SyntheticStreamTick);
+        assert_eq!(
+            model.pane(PaneId::ClaudeUi).synthetic_line_sequence(),
+            sequence_before + 1
+        );
+
+        let attached = model.dispatch(ShellAction::AttachPanel(PaneId::ClaudeUi));
+        assert!(attached.changed);
+        assert!(model.is_panel_attached(PaneId::ClaudeUi));
+        assert_eq!(model.selected_pane(), PaneId::ClaudeUi);
+        assert_eq!(model.counters().panel_detaches, 1);
+        assert_eq!(model.counters().panel_attaches, 1);
+    }
+
+    #[test]
+    fn detaching_selected_panel_prefers_the_next_sibling_then_previous() {
+        let mut model = ShellModel::default();
+        model.dispatch(ShellAction::SelectPane(PaneId::PiDocs));
+        model.dispatch(ShellAction::DetachPanel(PaneId::PiDocs));
+        assert_eq!(model.selected_pane(), PaneId::Architecture);
+
+        model.dispatch(ShellAction::SelectPane(PaneId::AcpPreview));
+        model.dispatch(ShellAction::DetachPanel(PaneId::AcpPreview));
+        assert_eq!(model.selected_pane(), PaneId::Architecture);
+    }
+
+    #[test]
+    fn the_last_attached_panel_cannot_be_detached() {
+        let mut model = ShellModel::default();
+        for pane in [
+            PaneId::ClaudeUi,
+            PaneId::PiDocs,
+            PaneId::Architecture,
+            PaneId::AcpPreview,
+        ] {
+            assert!(model.dispatch(ShellAction::DetachPanel(pane)).changed);
+        }
+        assert_eq!(model.attached_pane_ids(), vec![PaneId::CodexRuntime]);
+        let blocked = model.dispatch(ShellAction::DetachPanel(PaneId::CodexRuntime));
+        assert!(!blocked.changed);
+        assert_eq!(model.attached_panel_count(), 1);
+        assert_eq!(model.counters().blocked_panel_detaches, 1);
+    }
+
+    #[test]
+    fn hidden_panels_cannot_take_keyboard_focus() {
+        let mut model = ShellModel::default();
+        let outcome = model.dispatch(ShellAction::SelectPane(PaneId::RuntimeReview));
+        assert!(!outcome.changed);
+        assert_eq!(model.selected_pane(), PaneId::CodexRuntime);
+
+        model.dispatch(ShellAction::DetachPanel(PaneId::ClaudeUi));
+        let moved = model.dispatch(ShellAction::MoveFocus(FocusDirection::Right));
+        assert_eq!(moved.selected_pane, PaneId::PiDocs);
+    }
+
     #[test]
     fn direct_selection_is_idempotent_and_counted_separately() {
         let mut model = ShellModel::default();
@@ -677,10 +854,10 @@ mod tests {
         );
         assert!(
             model
-                .dispatch(ShellAction::SelectPane(PaneId::RuntimeReview))
+                .dispatch(ShellAction::SelectPane(PaneId::AcpPreview))
                 .changed
         );
-        assert_eq!(model.selected_pane(), PaneId::RuntimeReview);
+        assert_eq!(model.selected_pane(), PaneId::AcpPreview);
         assert_eq!(model.counters().direct_selections, 1);
         assert_eq!(model.counters().events_dispatched, 2);
         assert_eq!(model.counters().state_changes, 1);
@@ -829,7 +1006,8 @@ mod tests {
     fn identical_action_replays_produce_identical_models_and_counters() {
         let actions = [
             ShellAction::MoveFocus(FocusDirection::Right),
-            ShellAction::MoveFocus(FocusDirection::Down),
+            ShellAction::DetachPanel(PaneId::PiDocs),
+            ShellAction::AttachPanel(PaneId::RuntimeReview),
             ShellAction::OpenCommandPalette,
             ShellAction::SetCommandPaletteQuery("workspace".into()),
             ShellAction::CloseCommandPalette,