feat(tiles): selections with the grid open focus tiles, and never collapse it by themselves

selectSession gets the tile branch, right after its "already active" early
return: a tiled session is focused in its tile (_selectTiledSession: an
activeSessionId change, the shared _refreshSessionPanels and xterm.focus(),
no cleanup, replay, resize or socket of the parked main terminal). Decision 1:
a USER-initiated pick of a session that is not tiled leaves the grid for the
single view (the grid is remembered), and so does an explicit leaveTiles; an
app-driven pick (auto) never collapses it. A followed #session= link passes
leaveTiles (navigation).

App-driven paths pick a tile instead of the first sessionOrder entry:

- closeSession on the focused tile focuses the neighbouring tile (next in grid
  order, else previous), captured before the await like wasActive, since the
  delete broadcast may already have removed the tile; the last tile closes the
  grid and falls back to the normal pick.
- A tiled session deleted elsewhere loses its tile and a neighbour takes focus
  with auto (the last one lands on the welcome screen as before); a close from
  this tab only drops the tile and leaves the follow-up to closeSession.
- A tiled session popped out to its own window leaves the grid.

Focus rules: pressing a tile is a human selection (pointerdown, never
preventDefault); Ctrl+Tab and Alt+[ / Alt+] cycle through the tiles; only a
human selection acknowledges an idle alert. Home leaves the grid (remembered);
killing every session closes it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -1471,7 +1471,9 @@ class CodemanApp {
       return false;
     }
     this._retireUrlSession();
-    this.selectSession(id, { auto: true });
+    // Following a link is navigation: a tiled id focuses its tile, any other
+    // leaves the tile grid for the single view (the grid is remembered).
+    this.selectSession(id, { auto: true, leaveTiles: true });
     return true;
   }
 
@@ -1610,6 +1612,9 @@ class CodemanApp {
    *  Full re-renders re-apply the class from this.detachedSessions. */
   _markDetached(id, on) {
     if (on) this.detachedSessions.add(id); else this.detachedSessions.delete(id);
+    // A popped-out session's window owns its PTY size now, so it leaves the
+    // tile grid (one place per session in this browser tab).
+    if (on && this._tileGrid?.has(id)) this.removeTile(id, { refocus: true, auto: true });
     const container = this.$('sessionTabs');
     const tab = container && container.querySelector(`.session-tab[data-id="${id}"]`);
     if (tab) tab.classList.toggle('detached', on);
@@ -7973,6 +7978,20 @@ class CodemanApp {
       if (userInitiated) this.markIdleAlertSeen(sessionId);
       return;
     }
+    // Tile grid open (tile-grid.js): a tiled session is FOCUSED in its tile and
+    // never loaded into the parked main terminal. Decision 1: only a USER-
+    // initiated pick of a session that is not tiled (or an explicit
+    // `leaveTiles`, a followed link) leaves the grid for the single view, the
+    // grid remembered for one-click return. An app-driven pick (`auto`) never
+    // collapses it.
+    if (this._tileGrid?.open) {
+      if (this._tileGrid.has(sessionId)) return this._selectTiledSession(sessionId, options);
+      if (options?.auto === true && !options?.leaveTiles) return;
+      this.closeTileGrid({ keepStored: true, reselect: false });
+      // The parked terminal still holds what it showed before the grid opened;
+      // with no active id, the switch below snapshots none of it.
+      this.activeSessionId = null;
+    }
     if (this.activeSessionId === sessionId && forceReload) {
       this.terminalBufferCache?.delete(sessionId);
       this._xtermSnapshots?.delete(sessionId);
@@ -8828,12 +8847,24 @@ class CodemanApp {
     // next session or dumped you on the home screen, depending on which path
     // won the race (both outcomes measured on one build, 2026-08-17).
     const wasActive = this.activeSessionId === sessionId;
+    // Tile grid open: the fallback is the NEIGHBOURING TILE, never the first
+    // sessionOrder entry (often not tiled, which would collapse the grid).
+    // Captured here for the same reason as wasActive: the SSE delete can remove
+    // the tile while the request is still in flight.
+    const grid = this._tileGrid;
+    const tileNeighborId = grid?.has(sessionId) ? window.CodemanTileGrid.tileNeighbor(grid.ids, sessionId) : null;
     this._closingSessions.add(sessionId);
     try {
       await this._apiDelete(`/api/sessions/${sessionId}?killMux=${killMux}`);
       this._cleanupSessionData(sessionId);
+      // The last tile leaving closes the grid (no reselect): the pick below runs.
+      if (grid?.has(sessionId)) this.removeTile(sessionId, { refocus: false });
 
-      if (wasActive) {
+      if (wasActive && grid?.open) {
+        // `auto`: the app chose this tile because the previous one went away.
+        const target = grid.has(tileNeighborId) ? tileNeighborId : grid.ids[0];
+        this._selectTiledSession(target, { auto: true });
+      } else if (wasActive) {
         this.activeSessionId = null;
         try { localStorage.removeItem('codeman-active-session'); } catch {}
         // Next tab in the user's own order, skipping ids the cleanup has not
@@ -8918,6 +8949,12 @@ class CodemanApp {
   }
 
   nextSession() {
+    // With the tile grid open, Ctrl+Tab and Alt+] cycle through the tiles.
+    if (this._tileGrid?.open) {
+      const id = window.CodemanTileGrid.cycleTile(this._tileGrid.ids, this.activeSessionId, 1);
+      if (id) this.selectSession(id);
+      return;
+    }
     if (this.sessionOrder.length <= 1) return;
 
     const currentIndex = this.sessionOrder.indexOf(this.activeSessionId);
@@ -8926,6 +8963,11 @@ class CodemanApp {
   }
 
   prevSession() {
+    if (this._tileGrid?.open) {
+      const id = window.CodemanTileGrid.cycleTile(this._tileGrid.ids, this.activeSessionId, -1);
+      if (id) this.selectSession(id);
+      return;
+    }
     if (this.sessionOrder.length <= 1) return;
 
     const currentIndex = this.sessionOrder.indexOf(this.activeSessionId);
@@ -8941,6 +8983,8 @@ class CodemanApp {
     // Going Home is choosing something else, so a `#session=<id>` link still
     // waiting for its session must not take the screen later.
     this._retireUrlSession();
+    // Home is a choice to leave the grid too; it is remembered for Tiles.
+    this.closeTileGrid?.({ keepStored: true, reselect: false });
     // Deselect active session and show welcome screen
     this.activeSessionId = null;
     try { localStorage.removeItem('codeman-active-session'); } catch {}
@@ -8991,6 +9035,8 @@ class CodemanApp {
 
     try {
       await this._apiDelete('/api/sessions');
+      // Every tiled session is gone: nothing left to remember or reselect.
+      this.closeTileGrid?.({ keepStored: false, reselect: false });
       this.sessions.clear();
       this.terminalBuffers.clear();
       this.terminalBufferCache.clear();