ci(tests): retry a Chromium browser run that loses a test iframe

.github/workflows/tests.yml

@@ -105,8 +105,10 @@ jobs:
   test-browsers:
     name: Test on Browsers (${{ matrix.browser }})
     runs-on: ubuntu-latest
-    timeout-minutes: 20
+    timeout-minutes: 25
     strategy:
+      # A flake in one browser must not cancel the others, or its cause is lost.
+      fail-fast: false
       matrix:
         browser: [edge, chrome, firefox]
 
@@ -119,8 +121,36 @@ jobs:
       - name: Setup
         uses: ./.github/actions/setup
 
+      # Chromium under the webdriverio provider sometimes loses a test iframe: the run either fails
+      # at once with "Cannot connect to the iframe" / "Failed to fetch dynamically imported module",
+      # or prints nothing more until it is killed. A whole run takes 1 to 3 minutes, so an attempt
+      # that times out, or fails with one of those two errors, is run again, up to three attempts.
+      # The iframes are lost when many test files load at once, so the retries run one file at a
+      # time. Any other failure, including a failing assertion, fails the step at once.
       - name: Run tests in ${{ matrix.browser }}
-        run: vp test --browser.enabled --browser.name=${{ matrix.browser }}
+        env:
+          BROWSER: ${{ matrix.browser }}
+        run: |
+          attempt() {
+            timeout "$1" vp test --browser.enabled --browser.name="$BROWSER" "${@:2}" 2>&1 | tee browser-tests.log
+            return "${PIPESTATUS[0]}"
+          }
+
+          lost_iframe() {
+            [ "$1" -eq 124 ] || grep -qE "Cannot connect to the iframe|Failed to fetch dynamically imported module" browser-tests.log
+          }
+
+          attempt 300 && exit 0
+          status=$?
+
+          for retry in 1 2; do
+            lost_iframe "$status" || exit "$status"
+            echo "::warning::The $BROWSER test run lost a test iframe (exit $status); retry $retry of 2, one file at a time."
+            attempt 420 --no-file-parallelism && exit 0
+            status=$?
+          done
+
+          exit "$status"
 
   test-safari:
     name: Test on Browsers (safari)