diff --git a/.github/workflows/viewerjs-check.yml b/.github/workflows/viewerjs-check.yml
index d5c83424..0b67b161 100644
--- a/.github/workflows/viewerjs-check.yml
+++ b/.github/workflows/viewerjs-check.yml
@@ -41,9 +41,13 @@ concurrency:
 # 7.3m-12.2m), because --shard splits by test count and the slow concept specs sort into shard 1. The page
 # workflows now run in three shards per browser with the limits unchanged; the per-shard times are
 # re-measured after three green main runs.
+# #563: coverage shards reached their 18-minute step limit three times on 2026-10-31 with no failing
+# test (PR #560 shard 2/3: 814 passed in 17.9m; queue run 41206495451 shard 1/3: 827 passed; PR #564
+# shard 2/3 twice), against recent green shards of 11m-17m. Coverage now runs in four shards with the
+# limits unchanged; the per-shard times are re-measured after three green main runs.
 jobs:
   frontend-coverage:
-    name: frontend coverage (shard ${{ matrix.shard }}/3)
+    name: frontend coverage (shard ${{ matrix.shard }}/4)
     runs-on: ubuntu-latest
     # Layout comparisons depend on the fonts each browser falls back to, so browser jobs run in the
     # same pinned Playwright image as local runs (tag = @playwright/test in tools/vendor/package-lock.json;
@@ -59,7 +63,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        shard: [1, 2, 3]
+        shard: [1, 2, 3, 4]
     steps:
       - uses: actions/checkout@v7
       # The image's own Python 3.12 and Node 24: setup-python/setup-node install the runner's toolcache
@@ -88,13 +92,13 @@ jobs:
       - run: npm ci --prefix tools/vendor --ignore-scripts
       # Shard 1 also runs the node units and the viewer suite, so the vendor pins and the tooling tests
       # run on the lightest shard instead.
-      - if: matrix.shard == 3
+      - if: matrix.shard == 4
         run: npm run --prefix tools/vendor check
-      - if: matrix.shard == 3
+      - if: matrix.shard == 4
         run: npm run --prefix tools/vendor test
-      - name: Page workflows with coverage, shard ${{ matrix.shard }}/3
+      - name: Page workflows with coverage, shard ${{ matrix.shard }}/4
         timeout-minutes: 18
-        run: npm run --prefix tools/vendor test:coverage:shard -- ${{ matrix.shard }}/3
+        run: npm run --prefix tools/vendor test:coverage:shard -- ${{ matrix.shard }}/4
       - name: Upload raw coverage
         uses: actions/upload-artifact@v7
         with:
@@ -136,13 +140,13 @@ jobs:
         with:
           pattern: coverage-raw-*
           path: ${{ runner.temp }}/coverage-raw
-      # coverage:merge only checks that raw files exist; merging two of three shards would report
+      # coverage:merge only checks that raw files exist; merging three of four shards would report
       # partial coverage, so a missing or empty shard artifact fails here.
       - name: Verify every shard's raw coverage arrived
         shell: bash
         env:
           STAGED_RAW: ${{ runner.temp }}/coverage-raw
-          EXPECTED_SHARDS: 3
+          EXPECTED_SHARDS: 4
         run: |
           found=0
           for shard in $(seq 1 "$EXPECTED_SHARDS"); do
diff --git a/TESTING.md b/TESTING.md
index 3d249a28..c345ef7a 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -74,14 +74,14 @@ V8 files it writes under `coverage/js/raw`:
 
 ```sh
 PM_TEST_WORKERS=8 npm run --prefix tools/vendor test:coverage            # whole run, one report
-npm run --prefix tools/vendor test:coverage:shard -- 1/3                 # node units, viewer suite, workflows 1/3
-npm run --prefix tools/vendor test:coverage:shard -- 2/3                 # workflows 2/3 (likewise 3/3)
+npm run --prefix tools/vendor test:coverage:shard -- 1/4                 # node units, viewer suite, workflows 1/4
+npm run --prefix tools/vendor test:coverage:shard -- 2/4                 # workflows 2/4 (likewise 3/4, 4/4)
 npm run --prefix tools/vendor coverage:merge                             # report and baseline check of every raw file
 npm run --prefix tools/vendor test:workflows -- --project webkit --shard 2/2
 ```
 
 `coverage:merge` refuses an empty raw directory; it does not count shards, so run
-every shard first (CI's merge job checks all three arrived). Measured on a 32-CPU
+every shard first (CI's merge job checks all four arrived). Measured on a 32-CPU
 machine in the pinned image (2026-10-21): `test:coverage` takes about 5 minutes
 with 8 workers and 24 minutes with 1; all four projects, 8 workers, each test 3
 times (6132 tests), 24 minutes. A CI-sized run (`docker run --cpus=4`,
@@ -189,14 +189,14 @@ check, so run it before pushing a new or deleted frontend file instead of
 waiting for the `frontend coverage` jobs.
 
 The `viewerjs` check runs on every PR, including backend-only changes. It is an
-aggregate job. It requires the three `frontend coverage (shard N/3)` jobs and
+aggregate job. It requires the four `frontend coverage (shard N/4)` jobs and
 `frontend coverage (merge)` to succeed on every event. The four
 `page workflows (firefox|webkit, shard N/2)` jobs run only in the merge queue,
 on `main` and on `workflow_dispatch` (see "Pull requests and the merge queue").
 `viewerjs` requires them to be skipped on a pull request and to succeed
 everywhere else. Any other failed, cancelled or skipped job fails it. Every browser job runs with `PM_TEST_WORKERS: 3` (the public
-runners have 4 vCPUs). Each coverage shard runs `test:coverage:shard -- N/3`
-(shard 1 also runs the node units and the viewer suite, shard 3 the vendor pins
+runners have 4 vCPUs). Each coverage shard runs `test:coverage:shard -- N/4`
+(shard 1 also runs the node units and the viewer suite, shard 4 the vendor pins
 and tooling tests) and uploads its raw V8 files as `coverage-raw-N` (kept 3
 days). The merge job downloads them, fails unless all three arrived with raw
 files and without clashing names, then runs `coverage:merge` (report plus
@@ -664,7 +664,7 @@ what runs:
 | `py3.13 on ubuntu-latest`, `py3.14 on ubuntu-latest` | not created | run |
 | `real ComfyUI compatibility` | skipped | runs |
 | `coverage gate (85%)`, `black and flake8`, `frontend modules`, `bandit (high severity)` | run | run |
-| `frontend coverage (shard N/3)`, `frontend coverage (merge)` (vendor pins and tooling tests on shard 3) | run | run |
+| `frontend coverage (shard N/4)`, `frontend coverage (merge)` (vendor pins and tooling tests on shard 4) | run | run |
 | `page workflows (firefox\|webkit, shard N/2)` | skipped | run |
 | `all tests`, `viewerjs` (gates) | run | run |
 | `perf budget` | skipped | push and dispatch only (not in the queue) |
diff --git a/tests/browser/workspace-layout-tags-system.spec.mjs b/tests/browser/workspace-layout-tags-system.spec.mjs
index 64f8f9c1..a35cf8b3 100644
--- a/tests/browser/workspace-layout-tags-system.spec.mjs
+++ b/tests/browser/workspace-layout-tags-system.spec.mjs
@@ -156,8 +156,9 @@ test('at 390px the window fits the screen, actions stay reachable and the page s
     expect(frame.bottom-frame.top).toBeLessThan(844);
     expect(frame.bottom-frame.top).toBeGreaterThanOrEqual(280);
     expect(await page.evaluate(()=>document.documentElement.scrollWidth)).toBeLessThanOrEqual(390);
-    // The last row's actions can be scrolled to inside the window and fit its width.
-    const last=rows(page).last().getByRole('button',{name:'Rename / merge',exact:true});
+    // The first page's last row's actions can be scrolled to inside the window and fit its width.
+    // Pinned to row 100: reaching it loads the next page (D183), and last() would then follow row 200.
+    const last=rows(page).nth(99).getByRole('button',{name:'Rename / merge',exact:true});
     await last.scrollIntoViewIfNeeded();await expect(last).toBeInViewport();
     const [button]=await boxes(last);const [after]=await boxes(tagWindow(page));
     expect(button.right).toBeLessThanOrEqual(after.right);
diff --git a/tests/vendor/ci-workflow.test.mjs b/tests/vendor/ci-workflow.test.mjs
index 9a2ab16a..ad81231f 100644
--- a/tests/vendor/ci-workflow.test.mjs
+++ b/tests/vendor/ci-workflow.test.mjs
@@ -129,11 +129,11 @@ test('Firefox/WebKit page workflows run only outside pull requests; coverage run
     }
 });
 
-test('coverage runs as three shards that each upload their raw coverage', () => {
+test('coverage runs as four shards that each upload their raw coverage', () => {
     const coverage = allJobs.get('frontend-coverage');
-    assert.deepEqual(matrixList(coverage, 'shard'), ['1', '2', '3']);
-    assert.match(coverage, /^ {4}name: frontend coverage \(shard \$\{\{ matrix\.shard \}\}\/3\)$/m);
-    assert.match(coverage, /npm run --prefix tools\/vendor test:coverage:shard -- \$\{\{ matrix\.shard \}\}\/3/);
+    assert.deepEqual(matrixList(coverage, 'shard'), ['1', '2', '3', '4']);
+    assert.match(coverage, /^ {4}name: frontend coverage \(shard \$\{\{ matrix\.shard \}\}\/4\)$/m);
+    assert.match(coverage, /npm run --prefix tools\/vendor test:coverage:shard -- \$\{\{ matrix\.shard \}\}\/4/);
     const upload = steps(coverage).find(step => /name: coverage-raw-\$\{\{ matrix\.shard \}\}/.test(step));
     assert.ok(upload, 'each shard uploads coverage-raw-N');
     assert.match(upload, /^ {10}path: coverage\/js\/raw\/?$/m);