fix(cmux-tui-core): a tab order change validates its pane, not every tab of it

cmux-tui/crates/cmux-tui-core/src/state/group_update_latency_tests.rs

@@ -140,6 +140,28 @@ fn timed(latencies: &mut BTreeMap<&'static str, Vec<f64>>, name: &'static str, r
     latencies.entry(name).or_default().push(started.elapsed().as_secs_f64() * 1000.0);
 }
 
+/// A chat tab's create validates the tabs it changed, not every tab of its pane: the pane's new order
+/// (`SetTabOrder`) moves positions only, which its set and contiguous-position checks cover, as a workspace
+/// order change already does. Checking every tab took about 5 queries per tab of the pane under the registry
+/// lock, which a person's next click waits on (this file's first test).
+#[test]
+fn a_chat_tab_create_validates_only_the_tabs_it_changed() {
+    use std::sync::atomic::Ordering::Relaxed;
+    let session = Session::new("chat-tab-validates");
+    let mux = session.open();
+    let sidebar = sidebar(&mux);
+    let workspace = workspace_slot(&mux, &sidebar.loose[0]);
+    for index in 0..30 {
+        open_chat_tab(&mux, workspace, format!("before-{index}"));
+    }
+    let before = crate::workspace_registry::resource_store::TABS_VALIDATED.load(Relaxed);
+    open_chat_tab(&mux, workspace, "the-person".into());
+    let validated =
+        crate::workspace_registry::resource_store::TABS_VALIDATED.load(Relaxed) - before;
+    mux.shutdown();
+    assert!(validated <= 4, "one chat tab create in a 31-tab pane validated {validated} tabs");
+}
+
 #[test]
 fn a_persons_requests_answer_while_busy_sessions_saturate_the_registry() {
     let session = Session::new("person-request-latency");

cmux-tui/crates/cmux-tui-core/src/workspace_registry/resource_store.rs

@@ -4248,9 +4248,12 @@ fn validate_touched_resource_invariants(
                     &mut browsers,
                 )?;
             }
-            ResourceChange::SetTabOrder { pane_id, tab_ids } => {
+            // An order change writes only the pane's tabs' positions: the set (set_resource_tab_order) and the
+            // contiguous positions (below) are its invariants, so it touches no tab's own rows, as
+            // `SetWorkspaceOrder` above. Validating every tab of the pane per create cost O(tabs in the pane):
+            // about 5 queries per tab, under the registry lock a person's next click waits on.
+            ResourceChange::SetTabOrder { pane_id, .. } => {
                 panes.insert(pane_id.to_string());
-                tabs.extend(tab_ids.iter().map(ToString::to_string));
             }
             ResourceChange::UpsertTerminal { public_id, .. }
             | ResourceChange::TombstoneTerminal { public_id, .. } => {
@@ -4579,7 +4582,15 @@ fn validate_touched_pane(transaction: &Transaction<'_>, pane_id: &str) -> anyhow
     Ok(())
 }
 
+/// Tabs [`validate_touched_tab`] checked in this test process (nextest runs each test alone): a commit's
+/// validation must be O(changed), never O(tabs in the pane).
+#[cfg(test)]
+pub(crate) static TABS_VALIDATED: std::sync::atomic::AtomicU64 =
+    std::sync::atomic::AtomicU64::new(0);
+
 fn validate_touched_tab(transaction: &Transaction<'_>, tab_id: &str) -> anyhow::Result<()> {
+    #[cfg(test)]
+    TABS_VALIDATED.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
     let stored = transaction
         .query_row(
             "SELECT pane_id, content_kind, content_id, deleted_revision