Make downstream tests conditional on fast tests passing

.github/workflows/downstream.yml

@@ -1,13 +1,9 @@
 name: Test downstream projects
 
+# Called from tests.yml once the fast test jobs have passed, so we don't spend
+# runner time on downstream projects when traitlets itself is broken.
 on:
-  push:
-    branches: ["main"]
-  pull_request:
-
-concurrency:
-  group: downstream-${{ github.ref }}
-  cancel-in-progress: true
+  workflow_call:
 
 jobs:
   ipython:

.github/workflows/tests.yml

@@ -139,6 +139,18 @@ jobs:
           python_package_manager: "uv pip"
       - uses: jupyterlab/maintainer-tools/.github/actions/test-sdist@v1
 
+  # Downstream projects are slow and only meaningful if traitlets itself is
+  # healthy, so hold them until every fast job has finished and passed.
+  downstream:
+    needs:
+      - tests
+      - test_minimum_versions
+      - test_lint
+      - test_docs
+      - test_prereleases
+      - test_sdist
+    uses: ./.github/workflows/downstream.yml
+
   check_links:
     runs-on: ubuntu-latest
     timeout-minutes: 10