fix: zombie terminals eating the quota; tab drop double-firing into two panes

- The per-session terminal quota counted exited zombie handles (shells
  closed or dropped on hosts without the close-frame protocol), so enough
  abandoned terminals permanently blocked new ones — the 'disconnected,
  reconnecting' loop. open() now sweeps the session's exited handles before
  the quota check, and the running instance's leftovers disappear on the
  next restart (disposeAll).
- Dragging a tab onto the strip's background fired the drop TWICE: the
  TabBar handled it (merge into this pane) and the event bubbled to the
  pane root, which ran its edge-zone handler again — producing two splits /
  a duplicated instance (up+down). The strip now stops propagation for
  drag-over and drop.
- Tests: +1 real-pty smoke (exited handle swept, new tab spawns under a
  quota of 1).

src/client/TabBar.tsx

@@ -64,12 +64,17 @@ export function TabBar(props: {
     <div
       className={clsx(css.tabBar, dragOver && css.tabBarDrop)}
       onDragOver={(event) => {
+        // The strip owns drops on itself (merge into this pane); stopping
+        // propagation keeps the pane root from also running its edge-zone
+        // handler on the same drop.
         event.preventDefault()
+        event.stopPropagation()
         setDragOver(true)
       }}
       onDragLeave={() => { setDragOver(false) }}
       onDrop={(event) => {
         event.preventDefault()
+        event.stopPropagation()
         setDragOver(false)
         const raw = event.dataTransfer.getData(TAB_DRAG_TYPE)
         const payload = parseDrag(raw)

src/pty-manager.ts

@@ -96,6 +96,11 @@ export class PtyManager {
     const existing = this.sessions.get(key)
     if (existing !== undefined && !existing.exited) return existing
     if (existing !== undefined) this.close(key)
+    // Zombie cleanup: a session's exited handles (shell closed, tab dropped
+    // on an old host without the close frame) must not eat the quota.
+    for (const [candidate, handle] of [...this.sessions]) {
+      if (handle.sessionId === sessionId && handle.exited) this.close(candidate)
+    }
     if (this.keysOf(sessionId).length >= this.maxPerSession) {
       throw new SidebarError('pty-error', `terminal limit reached (${this.maxPerSession}) for this session`, 400)
     }

tests/smoke.spec.ts

@@ -87,6 +87,25 @@ describe('host plugin smoke', () => {
     }
   })
 
+  it('pty manager: exited zombie handles do not consume the quota', async () => {
+    const manager = new PtyManager(defaultShell(), 1)
+    try {
+      const first = manager.open('s3', 't1', process.cwd(), 80, 24)
+      first.pty.write('exit\r')
+      const deadline = Date.now() + 5000
+      while (!first.exited && Date.now() < deadline) {
+        await new Promise(resolve => setTimeout(resolve, 100))
+      }
+      expect(first.exited).toBe(true)
+      // Quota is 1; the exited handle is swept, so a NEW tab can still spawn.
+      const second = manager.open('s3', 't2', process.cwd(), 80, 24)
+      expect(second.exited).toBe(false)
+      expect(manager.keysOf('s3')).toHaveLength(1)
+    } finally {
+      manager.disposeAll()
+    }
+  })
+
   it('pty manager: a reconnect within the grace period cancels the pending close', async () => {
     const manager = new PtyManager(defaultShell(), 3)
     try {