diff --git a/tests/render_harness.py b/tests/render_harness.py
index d750f9c1b..29c5f1ce3 100644
--- a/tests/render_harness.py
+++ b/tests/render_harness.py
@@ -1340,6 +1340,27 @@ def new_page(**kwargs):
     return SimpleNamespace(new_page=new_page)
 
 
+def margins_laid_out(page):
+    """Run the margin layout the page has scheduled, so a geometry read follows it.
+
+    The margin's own geometry owner answers a width change through a ResizeObserver on
+    the body and a `requestAnimationFrame`, which is one whole rendering turn later than
+    the resize event `resized` waits for. A read taken in between is a read of the rows
+    at the width they have just left: on a page narrowed to exactly what its residents
+    need, the widest margin row was still at its roomy width and hung 24px past the
+    window — but only on the runs where the frame had not landed yet, which is why the
+    same probe condensed on one run and not the next.
+
+    The pending frame is not a fact to wait a frame for (`tests/CLAUDE.md`, "a fixed
+    number of animation frames only guesses"), so the work is run instead of guessed at.
+    Whether the observer schedules it at all is `test_render_margin.py`'s subject, not
+    that of a test reading the layout it produces."""
+    page.evaluate(
+        "() => import('/runtime/margin-layout.js')"
+        ".then(({layoutMarginRows}) => layoutMarginRows())"
+    )
+
+
 def panel_settled(page, open=True):
     """Wait for the panel to reach `open` and the page to finish making room for it.
 
diff --git a/tests/render_support.py b/tests/render_support.py
index 6136af34b..a012a2fe0 100644
--- a/tests/render_support.py
+++ b/tests/render_support.py
@@ -309,6 +309,7 @@
     hold_selection,
     key_line,
     leaf_page,
+    margins_laid_out,
     navigate,
     nudge,
     open_page,
@@ -600,6 +601,7 @@
     "live_leaf",
     "live_url",
     "live_watcher",
+    "margins_laid_out",
     "mark_edges",
     "mark_point",
     "mark_shows_beside_composer",
diff --git a/tests/test_render_margin.py b/tests/test_render_margin.py
index c28f2527a..748522985 100644
--- a/tests/test_render_margin.py
+++ b/tests/test_render_margin.py
@@ -15,6 +15,7 @@
     compare_with,
     leaf_page,
     live_url,
+    margins_laid_out,
     open_page,
     panel_settled,
     resized,
@@ -1028,9 +1029,7 @@ def offset():
     page.evaluate(
         "() => document.scrollingElement.scrollBy({top: 320, behavior: 'instant'})"
     )
-    page.evaluate(
-        "() => import('/runtime/margin-layout.js').then(({layoutMarginRows}) => layoutMarginRows())"
-    )
+    margins_laid_out(page)
     assert offset() == pytest.approx(before, abs=1)
 
     assert errors == []
diff --git a/tests/test_render_pages.py b/tests/test_render_pages.py
index 5221db0e2..5870abf95 100644
--- a/tests/test_render_pages.py
+++ b/tests/test_render_pages.py
@@ -52,6 +52,7 @@
     composer_quote,
     leaf_page,
     live_url,
+    margins_laid_out,
     nudge,
     open_page,
     page_registry,
@@ -2036,6 +2037,7 @@ def test_a_left_sidebar_uses_the_margin_until_the_page_needs_it_back(browser, se
     }"""
 
     resized(page, 1400, 900)
+    margins_laid_out(page)
     roomy = page.evaluate(reading)
     assert roomy["strip"] == 264
     assert roomy["float"] == "left" and roomy["position"] == "sticky"
@@ -2069,6 +2071,7 @@ def test_a_left_sidebar_uses_the_margin_until_the_page_needs_it_back(browser, se
     # root scrollport holds outside the container query's own width.
     exact = roomy["strip"] + 720 + roomy["rail"]
     resized(page, math.ceil(exact + roomy["viewportWidth"] - roomy["pageWidth"]), 900)
+    margins_laid_out(page)
     tighter = page.evaluate(reading)
     assert exact <= tighter["pageWidth"] <= exact + 1, (
         f"the narrowed page is not the width the residents and column need: {tighter}"