diff --git a/tests/mutations/r2b.toml b/tests/mutations/r2b.toml
index 877f01d..8b0f494 100644
--- a/tests/mutations/r2b.toml
+++ b/tests/mutations/r2b.toml
@@ -262,3 +262,23 @@ old = '''
   .blurred-thumb{filter:blur(16px)}'''
 new = '''
   .blurred-thumb{filter:blur(0px)}'''
+
+# ---- the flake: the test browser has no internet (positive control per file)
+
+[[mutation]]
+label = "the flow test browser can reach the internet (Google Fonts can stall networkidle)"
+file = "tests/test_flow_browser.py"
+test = "tests/test_flow_browser.py::test_the_test_browser_has_no_internet"
+old = '''
+            b = pw.chromium.launch(args=OFFLINE)'''
+new = '''
+            b = pw.chromium.launch()'''
+
+[[mutation]]
+label = "the embed test browser can reach the internet"
+file = "tests/test_embed_browser.py"
+test = "tests/test_embed_browser.py::test_the_test_browser_has_no_internet"
+old = '''
+            b = pw.chromium.launch(args=OFFLINE)'''
+new = '''
+            b = pw.chromium.launch()'''
diff --git a/tests/test_embed_browser.py b/tests/test_embed_browser.py
index 162146b..2b18fa4 100644
--- a/tests/test_embed_browser.py
+++ b/tests/test_embed_browser.py
@@ -31,11 +31,22 @@ playwright_api = pytest.importorskip(
 )
 
 
+# NO INTERNET for the test browser. Every Booth page asks fonts.googleapis.com
+# for its faces, and "networkidle" waits for that request — so a stalled request
+# to Google hung the page until goto's 30s timeout, the failure mode of the
+# full-suite flake (Page.goto timeouts in tests far apart in one run; a stalled
+# font request reproduces it exactly). Whether that was THE cause is unproven;
+# a test that depends on Google being reachable is wrong regardless. Every
+# hostname but 127.0.0.1 now fails DNS at once, and the pages fall back to the
+# system stacks the tokens declare. Positive control: test_*_has_no_internet.
+OFFLINE = ["--host-resolver-rules=MAP * ~NOTFOUND , EXCLUDE 127.0.0.1"]
+
+
 @pytest.fixture(scope="module")
 def browser():
     with playwright_api.sync_playwright() as pw:
         try:
-            b = pw.chromium.launch()
+            b = pw.chromium.launch(args=OFFLINE)
         except Exception as exc:  # noqa: BLE001 - any launch failure is a skip
             pytest.skip(f"no usable chromium: {exc}")
         yield b
@@ -638,3 +649,24 @@ def test_the_keyboard_flag_actually_submits(browser, live):
 
     assert flagged == 1, f"the f key flagged {flagged} items, expected 1"
     assert survived, "the flag reloaded the page; in-place judgment must not"
+
+
+def test_the_test_browser_has_no_internet(browser, live):
+    """Positive control for OFFLINE (booth-dev's ask: see the fix in force,
+    don't assume it). An external host fails at once, and a Booth page — whose
+    fonts are external — still goes idle in well under the goto timeout."""
+    base, root = live
+    (root / "g").mkdir()
+    page = browser.new_page()
+    t = time.time()
+    with pytest.raises(Exception) as err:
+        page.goto("https://fonts.googleapis.com/css2?family=IBM+Plex+Sans", timeout=10000)
+    external = time.time() - t
+    page.close()
+    page = browser.new_page()
+    t = time.time()
+    page.goto(f"{base}/b/g/", wait_until="networkidle")
+    local = time.time() - t
+    page.close()
+    assert "ERR_NAME_NOT_RESOLVED" in str(err.value) and external < 3, (str(err.value)[:80], external)
+    assert local < 10, local
diff --git a/tests/test_flow_browser.py b/tests/test_flow_browser.py
index bbeb127..7757147 100644
--- a/tests/test_flow_browser.py
+++ b/tests/test_flow_browser.py
@@ -24,11 +24,22 @@ playwright_api = pytest.importorskip("playwright.sync_api", reason="playwright i
 PNG = b"\x89PNG\r\n\x1a\n"
 
 
+# NO INTERNET for the test browser. Every Booth page asks fonts.googleapis.com
+# for its faces, and "networkidle" waits for that request — so a stalled request
+# to Google hung the page until goto's 30s timeout, the failure mode of the
+# full-suite flake (Page.goto timeouts in tests far apart in one run; a stalled
+# font request reproduces it exactly). Whether that was THE cause is unproven;
+# a test that depends on Google being reachable is wrong regardless. Every
+# hostname but 127.0.0.1 now fails DNS at once, and the pages fall back to the
+# system stacks the tokens declare. Positive control: test_*_has_no_internet.
+OFFLINE = ["--host-resolver-rules=MAP * ~NOTFOUND , EXCLUDE 127.0.0.1"]
+
+
 @pytest.fixture(scope="module")
 def browser():
     with playwright_api.sync_playwright() as pw:
         try:
-            b = pw.chromium.launch()
+            b = pw.chromium.launch(args=OFFLINE)
         except Exception as exc:  # noqa: BLE001 - any launch failure is a skip
             pytest.skip(f"no usable chromium: {exc}")
         yield b
@@ -773,3 +784,24 @@ def test_the_review_and_doc_top_bars_fit_a_phone(browser, live):
             '.vbar button, .vbar .vbtn')].filter(e => e.offsetParent).map(e => e.getBoundingClientRect().height)) - 40""")
         page.close()
     assert all(v <= 0 for v in over.values()), over
+
+
+def test_the_test_browser_has_no_internet(browser, live):
+    """Positive control for OFFLINE (booth-dev's ask: see the fix in force,
+    don't assume it). An external host fails at once, and a Booth page — whose
+    fonts are external — still goes idle in well under the goto timeout."""
+    base, root = live
+    (root / "g").mkdir()
+    page = browser.new_page()
+    t = time.time()
+    with pytest.raises(Exception) as err:
+        page.goto("https://fonts.googleapis.com/css2?family=IBM+Plex+Sans", timeout=10000)
+    external = time.time() - t
+    page.close()
+    page = browser.new_page()
+    t = time.time()
+    page.goto(f"{base}/b/g/", wait_until="networkidle")
+    local = time.time() - t
+    page.close()
+    assert "ERR_NAME_NOT_RESOLVED" in str(err.value) and external < 3, (str(err.value)[:80], external)
+    assert local < 10, local