Restore unpinned desktop stream scroll

tests/test_issue5637_mobile_scroll_clamp_jumpback.py

@@ -94,17 +94,19 @@ def test_content_visibility_scoped_to_user_rows():
     )
 
 
-def test_restore_same_frame_holds_position_for_unpinned_reader():
-    """The absolute-top fallback must be skipped for an unpinned reader.
+def test_restore_same_frame_holds_position_for_unpinned_touch_reader():
+    """The absolute-top fallback must be skipped when native anchoring can hold.
 
     When the reader is scrolled up (userUnpinned) and the anchor restore failed,
     _restoreMessageScrollSnapshotSameFrame must NOT write an absolute snapshot.top
     (it goes stale and nudges the viewport backward as scrollHeight grows, #5637).
-    It should hold position and return before the el.scrollTop write.
+    It should hold position and return before the el.scrollTop write. Desktop
+    intentionally falls through to the explicit fallback because it has no
+    native overflow-anchor layer.
     """
     fn = _restore_same_frame_fn()
-    assert "snapshot.userUnpinned===true&&snapshot.pinned!==true" in fn, (
-        "The fallback must guard the unpinned/anchor-failed case (#5637)."
+    assert "snapshot.userUnpinned===true&&snapshot.pinned!==true&&_fbTouchHold" in fn, (
+        "The fallback must guard the unpinned/anchor-failed touch case (#5637)."
     )
     # The guard must sit BEFORE the absolute-top scrollTop write and short-circuit it.
     guard_idx = fn.index("snapshot.userUnpinned===true&&snapshot.pinned!==true")