ci(js-tests): shard desktop check:test:ui 3 ways — merge-group critical path

.github/actions/node-workspace/action.yml

@@ -0,0 +1,54 @@
+name: Node workspace setup
+description: >-
+  Node 26 + npm 12 + the lockfile-keyed installed node_modules tree, shared by
+  every job in js-tests.yml. One definition so the jobs cannot drift onto
+  different cache keys (a drift turns one job's 8s restore into a full
+  `npm ci`). Run actions/checkout first.
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+      with:
+        node-version: 26
+        cache: npm
+
+    - name: grab npm 12
+      shell: bash
+      run: |
+        # No-op once the bundled npm is already 12.x — saves ~5-15s/job and
+        # keeps the installed major aligned with the npm12 cache-key tag.
+        npm --version | grep -q '^12\.' || npm i -g npm@12
+
+    # The ``cache: npm`` option of ``setup-node`` caches only the ~/.npm
+    # tarball cache. The job then extracts the full workspace node_modules
+    # again and runs the postinstalls again, which includes the Electron
+    # binary fetch. This caches the installed tree itself, keyed on the
+    # lockfile, and skips ``npm ci`` on an exact hit. There are no
+    # restore-keys: a partial hit leaves a stale tree, so anything other
+    # than an exact lockfile match reinstalls from the start.
+    #
+    # This install runs WITH scripts, so the tree holds the postinstall
+    # artifacts. The postinstall of electron unpacks its binary into
+    # node_modules/electron/dist, which is inside the cached tree.
+    #
+    # The ~/.cache/electron download cache stays out of the key on purpose.
+    # ``npm ci`` is skipped on a hit, so nothing reads that cache. It only
+    # makes the archive larger.
+    - name: Restore node_modules
+      id: node-modules-cache
+      uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809 # v4.2.4
+      with:
+        path: |
+          node_modules
+          apps/*/node_modules
+          ui-tui/node_modules
+          ui-tui/packages/*/node_modules
+          tests-js/node_modules
+          web/node_modules
+        key: node-modules-scripts-${{ runner.os }}-node26-npm12-${{ hashFiles('package-lock.json') }}
+
+    - uses: ./.github/actions/retry
+      if: steps.node-modules-cache.outputs.cache-hit != 'true'
+      with:
+        command: npm ci

.github/scripts/run-workspace-checks.mjs

@@ -15,6 +15,7 @@
 //
 // This also runs on a laptop: `node .github/scripts/run-workspace-checks.mjs`.
 // `--concurrency N` sets the limit. `--list` prints the units and exits.
+// `--skip <pkg>::<script>` leaves one unit out (repeatable).
 
 import { execFileSync, spawn } from 'node:child_process'
 import { availableParallelism } from 'node:os'
@@ -76,7 +77,20 @@ function runUnit(unit) {
 
 async function main() {
   const argv = process.argv.slice(2)
-  const units = discoverUnits()
+  let units = discoverUnits()
+
+  // `--skip <pkg>::<script>` (repeatable) drops a unit that another CI job
+  // runs instead (js-tests.yml shards apps/desktop check:test:ui). A skip
+  // that matches nothing is an error: after a rename the suite would
+  // otherwise run in neither job and everything would still report green.
+  const skips = argv.flatMap((a, i) => (a === '--skip' ? [argv[i + 1]] : []))
+  for (const skip of skips) {
+    if (!units.some((u) => `${u.pkg}::${u.script}` === skip)) {
+      console.error(`::error::--skip ${skip} matches no workspace check unit — refusing to guess.`)
+      process.exit(1)
+    }
+  }
+  units = units.filter((u) => !skips.includes(`${u.pkg}::${u.script}`))
 
   if (units.length === 0) {
     console.error(

.github/workflows/js-tests.yml

@@ -51,49 +51,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
-        with:
-          node-version: 26
-          cache: npm
-
-      - name: grab npm 12
-        run: |
-          # No-op once the bundled npm is already 12.x — saves ~5-15s/job and
-          # keeps the installed major aligned with the npm12 cache-key tag.
-          npm --version | grep -q '^12\.' || npm i -g npm@12
-
-      # The ``cache: npm`` option of ``setup-node`` caches only the ~/.npm
-      # tarball cache. The job then extracts the full workspace node_modules
-      # again and runs the postinstalls again, which includes the Electron
-      # binary fetch. This caches the installed tree itself, keyed on the
-      # lockfile, and skips ``npm ci`` on an exact hit. There are no
-      # restore-keys: a partial hit leaves a stale tree, so anything other
-      # than an exact lockfile match reinstalls from the start.
-      #
-      # This install runs WITH scripts, so the tree holds the postinstall
-      # artifacts. The postinstall of electron unpacks its binary into
-      # node_modules/electron/dist, which is inside the cached tree.
-      #
-      # The ~/.cache/electron download cache stays out of the key on purpose.
-      # ``npm ci`` is skipped on a hit, so nothing reads that cache. It only
-      # makes the archive larger.
-      - name: Restore node_modules
-        id: node-modules-cache
-        uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809 # v4.2.4
-        with:
-          path: |
-            node_modules
-            apps/*/node_modules
-            ui-tui/node_modules
-            ui-tui/packages/*/node_modules
-            tests-js/node_modules
-            web/node_modules
-          key: node-modules-scripts-${{ runner.os }}-node26-npm12-${{ hashFiles('package-lock.json') }}
-
-      - uses: ./.github/actions/retry
-        if: steps.node-modules-cache.outputs.cache-hit != 'true'
-        with:
-          command: npm ci
+      - uses: ./.github/actions/node-workspace
 
       # Every check runs at the same time. The step fails only after all of
       # them finish. There are two reasons this is not ``npm run --ws check``.
@@ -112,5 +70,33 @@ jobs:
       # ``check:*`` script needs no change here. An empty list is an error and
       # not an empty run, because an empty run reports green after it checks
       # nothing.
+      #
+      # ``--skip`` removes the one unit the ``ui-tests`` job below runs
+      # sharded. The script errors if a skip matches no unit, so a rename of
+      # that script cannot silently drop the suite from both jobs.
       - name: Run all workspace checks
-        run: node .github/scripts/run-workspace-checks.mjs
+        run: node .github/scripts/run-workspace-checks.mjs --skip 'apps/desktop::check:test:ui'
+
+  ui-tests:
+    # apps/desktop ``check:test:ui`` was the whole critical path of this
+    # workflow and of the merge group: merge_group run 36217656078 spent
+    # 656.7s in it (685 files / 6774 tests) while the other 9 checks were done
+    # by 04:27, and it pushed the run to 11m26s. vitest's own breakdown was
+    # import 747s + environment 644s + tests 329s of worker time — per-file
+    # jsdom/import cost across 3 workers, so it splits linearly across
+    # runners. ``--shard`` partitions the FILE list deterministically; every
+    # file runs in exactly one shard, each shard in its own jsdom workers as
+    # before. Same command as ``check:test:ui`` (``npm run test:ui``) plus the
+    # shard flag.
+    name: UI tests (shard ${{ matrix.shard }}/3)
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2, 3]
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: ./.github/actions/node-workspace
+      - name: vitest ui project, shard ${{ matrix.shard }}/3
+        run: npm run --prefix apps/desktop test:ui -- --shard=${{ matrix.shard }}/3