fix(desktop): closing main in a side-by-side layout promotes the neighboring session instead of spawning a fresh draft (#88924)

nextSessionTileForWorkspace() only walked tabs stacked WITH the workspace
tab. In a side-by-side layout (session tiles in their own zones — db's
three-pane report), the walk found nothing, closeWorkspaceTab() skipped
promotion, and requestFreshSession() dropped main to a new draft: closing
a pane read as 'it gave me a new session'. A tile in any zone now promotes
(its zone collapses via the tile close), so Close is also the path from N
panes back to 1. Ghost panes whose tile is gone still never promote.

apps/desktop/src/store/session-states.test.ts

@@ -11,6 +11,7 @@ import {
   blankDraftTile,
   focusedSessionNeedsRoute,
   markSelectionRestore,
+  nextSessionTileForWorkspace,
   orderTilesByTree,
   releaseSessionTranscript,
   resetTileRuntimeBindings,
@@ -111,6 +112,47 @@ describe('selectionHomesToWorkspace', () => {
   })
 })
 
+describe('nextSessionTileForWorkspace (⌘W promotion source)', () => {
+  afterEach(() => {
+    $layoutTree.set(null)
+    $sessionTiles.set([])
+  })
+
+  it('prefers a tile stacked WITH the workspace tab (nearest-out)', () => {
+    $layoutTree.set(group(['workspace', tilePane('a'), tilePane('b')], { active: 'workspace', id: 'main' }))
+    $sessionTiles.set([tile('a'), tile('b')])
+
+    expect(nextSessionTileForWorkspace()).toBe('a')
+  })
+
+  it('side-by-side layout: a tile in ANOTHER zone still promotes instead of dropping main to a fresh draft (#88924)', () => {
+    // main zone holds only the workspace; the session tile lives in its own
+    // zone beside it — db's three-pane report shape.
+    $layoutTree.set(
+      split('row', [
+        group(['workspace'], { active: 'workspace', id: 'main' }),
+        group([tilePane('side')], { active: tilePane('side'), id: 'right' })
+      ])
+    )
+    $sessionTiles.set([tile('side')])
+
+    expect(nextSessionTileForWorkspace()).toBe('side')
+  })
+
+  it('returns null when no live tile exists anywhere in the tree', () => {
+    $layoutTree.set(
+      split('row', [
+        group(['workspace'], { active: 'workspace', id: 'main' }),
+        group([tilePane('stale')], { active: tilePane('stale'), id: 'right' })
+      ])
+    )
+    // Pane persisted in the tree but its tile is gone — must not promote a ghost.
+    $sessionTiles.set([])
+
+    expect(nextSessionTileForWorkspace()).toBe(null)
+  })
+})
+
 describe('boot-restore selection homing (⌘R tab persistence)', () => {
   const mainGroup = () => group(['workspace', tilePane('t')], { active: tilePane('t'), id: 'main' })
 

apps/desktop/src/store/session-states.ts

@@ -818,6 +818,17 @@ export function nextSessionTileForWorkspace(): null | string {
     }
   }
 
+  // Nothing stacked WITH main — but a session tile in another zone can still
+  // shift in. Without this, closing main in a side-by-side layout skipped
+  // promotion entirely and dropped to a fresh "New session" draft, which read
+  // as "closing a pane gave me a new session" (#88924). Promoting the tile
+  // also collapses its zone, so Close is how a multi-pane layout shrinks.
+  for (const tile of tiles) {
+    if (tree && findGroupOfPane(tree, `${TILE_PANE_PREFIX}${tile.storedSessionId}`)) {
+      return tile.storedSessionId
+    }
+  }
+
   return null
 }