Phase 2: drag-/right-click-a-pane-to-new-window

Right-click any pane's title bar → "Move to new window" pops it into a
fresh tiletopia window with its PTY intact. Same Tauri process; the
PtyManager is shared, so the existing PaneId stays valid and Tauri 2's
process-wide event routing keeps pane://{id}/data flowing into the new
window's XtermPane.

Mechanism (Rust-side, plan-agent's main correction over my draft):
- pty.rs: PtyManager.transferring is a per-pane refcount; kill_pane
  becomes a no-op while it's >0. Source window's React unmount calls
  kill_pane → silently dropped while in flight; target window's
  claim_pane decrements after it has subscribed.
- window_state.rs: per-window workspaces snapshot map +
  debounced-by-tokio aggregate save. Each window pushes its tabs via
  push_window_workspaces; backend writes the merged
  { version: 2, workspaces: [...] } envelope. Non-main windows have
  their entries dropped on CloseRequested so closing a detached window
  discards its tabs (Chrome-style).
- commands: mark_pane_transferring, claim_pane, get_pane_ring (base64
  scrollback ring snapshot), create_pane_window, take_pending_window_init,
  push_window_workspaces.

Frontend:
- XtermPane gets `existingPaneId?: PaneId`: skip spawn, replay ring
  snapshot via term.write before attaching the live data listener,
  resize PTY to this window's grid, claim_pane. Scrollback replay was
  the plan agent's other ship-in-v1 call — without it a transferred
  Claude session looks blank until next prompt repaint.
- LeafPane: onContextMenu opens a fixed-positioned "Move to new
  window" popover. Esc / outside-click dismiss.
- orchestration adds moveToNewWindow + getInitialPaneIdFor; App owns a
  one-shot transferredPaneIdsRef cleared in registerPaneId.
- App mount branches on getCurrentWebviewWindow().label: main loads
  workspace.json as before; non-main calls take_pending_window_init
  and builds a singleton workspace around the adopted leaf.
- MCP mirror + onMcpRequest only run in main (paneIdByLeafRef is per-
  window; Claude sees the main window's current tab as the single
  workspace surface).

pnpm check (tsc -b) clean. 79/79 vitest pass. Rust side authored in
WSL; cargo build needs verification on Windows host before this is
runnable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -6,11 +6,13 @@ mod hosts;
 mod mcp;
 mod mcp_policy;
 mod pty;
+mod window_state;
 
 use std::sync::Arc;
 
 use crate::mcp::{McpServerHandle, McpState, PendingActions};
 use crate::pty::PtyManager;
+use crate::window_state::{PendingInits, WindowsState, MAIN_WINDOW_LABEL};
 
 pub fn run() {
     let _ = tracing_subscriber::fmt()
@@ -40,6 +42,15 @@ pub fn run() {
     // Pending action registry — separate managed state so mcp_action_reply can
     // grab it without needing to lock McpState or reach into TileService.
     let pending_actions: Arc<PendingActions> = Arc::new(PendingActions::default());
+    // Cross-window workspace aggregator: every window pushes its tab list
+    // here; backend debounces + writes the merged envelope to workspace.json.
+    let windows_state: Arc<WindowsState> = Arc::new(WindowsState::default());
+    // Pane-transfer pending-init registry: source window stashes a payload
+    // keyed by the new window's label; target window pulls it during mount.
+    let pending_inits: Arc<PendingInits> = Arc::new(PendingInits::default());
+
+    let windows_state_for_event = Arc::clone(&windows_state);
+    let pending_inits_for_event = Arc::clone(&pending_inits);
 
     tauri::Builder::default()
         .plugin(tauri_plugin_clipboard_manager::init())
@@ -48,12 +59,34 @@ pub fn run() {
         .manage(mcp_state)
         .manage(McpServerHandle::default())
         .manage(pending_actions)
+        .manage(windows_state)
+        .manage(pending_inits)
+        .on_window_event(move |window, event| {
+            // When a non-main window closes, drop its workspaces from the
+            // aggregator AND any unconsumed pending-init payload so neither
+            // resurrect on next launch. Matches Chrome-style "closing a
+            // detached window discards its tabs" intent.
+            if let tauri::WindowEvent::CloseRequested { .. } = event {
+                let label = window.label().to_string();
+                if label != MAIN_WINDOW_LABEL {
+                    pending_inits_for_event.by_label.lock().remove(&label);
+                    windows_state_for_event
+                        .forget(window.app_handle().clone(), &label);
+                }
+            }
+        })
         .invoke_handler(tauri::generate_handler![
             commands::list_distros,
             commands::spawn_pane,
             commands::write_to_pane,
             commands::resize_pane,
             commands::kill_pane,
+            commands::mark_pane_transferring,
+            commands::claim_pane,
+            commands::get_pane_ring,
+            commands::create_pane_window,
+            commands::take_pending_window_init,
+            commands::push_window_workspaces,
             commands::save_workspace,
             commands::load_workspace,
             commands::list_ssh_hosts,