fix(layout): gate off-track bump on the feeding run's actual flow span

src/nf_metro/layout/phases/off_track.py

@@ -1501,6 +1501,38 @@ def _place(st: Station, candidate: float, bump_dir: float) -> None:
     return lift_extreme, opp_extreme
 
 
+def _feeding_run_spans_flow_coord(
+    graph: MetroGraph,
+    trunk_st: Station,
+    off_flow: float,
+    flow_axis: str,
+    in_section: set[str],
+) -> bool:
+    """Whether some run feeding *trunk_st* actually spans ``off_flow``.
+
+    The run drawn into ``trunk_st`` starts at its own in-section
+    predecessor(s) (``in_section`` -- station and port ids of the section
+    *trunk_st* belongs to), not at the icon.  A predecessor whose flow
+    coordinate also lies downstream of ``off_flow`` means that run never
+    reaches back far enough to cross the icon's column, even though
+    ``trunk_st`` itself is downstream.  With no in-section predecessor, be
+    conservative and treat the trunk station as contesting.
+    """
+    preds = [e.source for e in graph.edges_to(trunk_st.id) if e.source in in_section]
+    if not preds:
+        return True
+    trunk_flow = getattr(trunk_st, flow_axis)
+    for pid in preds:
+        pred_st = graph.stations.get(pid)
+        if pred_st is None:
+            continue
+        pred_flow = getattr(pred_st, flow_axis)
+        lo, hi = min(pred_flow, trunk_flow), max(pred_flow, trunk_flow)
+        if lo - SAME_COORD_TOLERANCE <= off_flow <= hi + SAME_COORD_TOLERANCE:
+            return True
+    return False
+
+
 def _bump_off_track_clear_of_trunks(
     graph: MetroGraph,
     off_st: Station,
@@ -1542,10 +1574,12 @@ def _bump_off_track_clear_of_trunks(
     MAX_STEPS = 6
 
     offset_step = resolve_offset_step(graph.track_gap)
+    in_section = set(section.station_ids) | section.port_ids
 
     # Find trunk stations in the same section whose line bundle crosses the
-    # icon's column (they lie downstream of the icon in flow, so the run
-    # feeding them passes the icon's flow coordinate).
+    # icon's column: downstream of the icon in flow is necessary but not
+    # sufficient, since the run feeding a downstream station may itself start
+    # further downstream than the icon and so never reach the icon's column.
     trunk_bands: list[float] = []
     for sid in section.station_ids:
         st2 = graph.stations.get(sid)
@@ -1557,6 +1591,10 @@ def _bump_off_track_clear_of_trunks(
             continue
         if flow_sign * (getattr(st2, flow_axis) - off_flow) <= SAME_COORD_TOLERANCE:
             continue
+        if not _feeding_run_spans_flow_coord(
+            graph, st2, off_flow, flow_axis, in_section
+        ):
+            continue
         # The line-track band spread across the cross axis at this trunk
         # station: each line sits at ``st2_cross + offset(line)``, bounded by
         # ``(n_lines - 1) * OFFSET_STEP`` centred on the station's cross coord.

tests/test_compensation_passes_idempotent.py

@@ -94,18 +94,6 @@
 # the X positions of two off-track sibling stations instead of reproducing
 # them, an order-sensitivity bug that has not been investigated further.
 #
-# ``topologies/paired_input_fan_branch_tree``'s "6.6"/"6.8" entries record a
-# real divergence that call order alone keeps benign.  Stage 6.18 seats the
-# branch-tree spur on the row an off-track output icon already occupies two
-# columns to its left.  ``_bump_off_track_clear_of_trunks`` then admits any
-# non-terminus trunk station merely downstream of the icon in flow and bumps
-# when that station's line band overlaps the icon's cross extent -- but the
-# seated branch is fed from a column downstream of the icon, so its inbound run
-# never reaches the icon's column.
-# The gate needs the feeding run's actual flow span, not just "downstream of the
-# icon" (#1566).  Stage 6.18 runs after the last reanchor, so the shipped
-# geometry keeps the tighter placement.
-#
 # Entries are removed only when the underlying stage genuinely becomes an
 # end-of-layout no-op; the assertions below fail loudly both on any new,
 # unregistered gap and on any registered gap that stops reproducing, so this
@@ -123,7 +111,6 @@
     "topologies/off_track_input_above_consumer": frozenset({"4.7"}),
     "topologies/out_of_section_retag_fan": frozenset({"4.7"}),
     "topologies/packed_multiline_serpentine_grid": frozenset({"4.7"}),
-    "topologies/paired_input_fan_branch_tree": frozenset({"6.6", "6.8"}),
     "topologies/rl_entry_right_exit_left": frozenset({"4.7"}),
     "topologies/rowmate_tb_side_entry_top_align_grow": frozenset({"4.7"}),
     "topologies/side_branch_ascent_label_strike": frozenset({"4.7"}),

tests/test_layout_invariants.py

@@ -90,6 +90,7 @@
     _tb_top_entry_drop_overshoot,
 )
 from nf_metro.layout.phases.off_track import (
+    _bump_off_track_clear_of_trunks,
     _cross_bbox_edges,
     _is_single_trunk_section,
     _off_track_anchor_of,
@@ -117,6 +118,7 @@
 from nf_metro.parser.mermaid import parse_metro_mermaid
 from nf_metro.parser.model import (
     BYPASS_V_PREFIX,
+    Edge,
     MetroGraph,
     PortSide,
     Section,
@@ -1270,6 +1272,52 @@ def test_single_line_dual_source_exit_shares_feeder_row():
     )
 
 
+def _off_track_bump_fixture(pred_x: float) -> tuple[MetroGraph, Station, Section]:
+    """A minimal section: an off-track icon, a trunk station downstream of
+    it in flow, and that trunk station's own in-section predecessor at
+    ``pred_x``.  The predecessor sits far off the icon's row so it never
+    contests on its own; only its flow coordinate matters here."""
+    section = Section(id="sec", name="Sec", station_ids=["icon", "pred", "trunk"])
+    graph = MetroGraph()
+    graph.sections["sec"] = section
+    icon = Station(
+        id="icon", label="", x=100.0, y=100.0, section_id="sec", off_track=True
+    )
+    pred = Station(id="pred", label="P", x=pred_x, y=500.0, section_id="sec")
+    trunk = Station(id="trunk", label="T", x=250.0, y=100.0, section_id="sec")
+    graph.stations["icon"] = icon
+    graph.stations["pred"] = pred
+    graph.stations["trunk"] = trunk
+    graph.edges = [Edge(source="pred", target="trunk", line_id="L1")]
+    return graph, icon, section
+
+
+def test_off_track_bump_gate_ignores_run_that_never_reaches_icon_column():
+    """A trunk station downstream of an off-track icon in flow only
+    contests the icon's row if its own feeding run's predecessor sits on
+    the near side of the icon.  A predecessor already downstream of the
+    icon means that run never reaches back across its column, so it must
+    not bump the icon out to a further row.
+    """
+    graph, icon, section = _off_track_bump_fixture(pred_x=150.0)  # downstream of icon
+    result = _bump_off_track_clear_of_trunks(
+        graph, icon, candidate=100.0, step=20.0, section=section, junction_ids=set()
+    )
+    assert result == pytest.approx(100.0)
+
+
+def test_off_track_bump_gate_bumps_when_feeding_run_spans_icon_column():
+    """A trunk station fed from a predecessor on the near side of the icon
+    genuinely has its run crossing the icon's column, so the icon must be
+    bumped clear of it.
+    """
+    graph, icon, section = _off_track_bump_fixture(pred_x=50.0)  # upstream of icon
+    result = _bump_off_track_clear_of_trunks(
+        graph, icon, candidate=100.0, step=20.0, section=section, junction_ids=set()
+    )
+    assert result != pytest.approx(100.0)
+
+
 @pytest.mark.parametrize("fixture", ALL_FIXTURES)
 def test_port_station_and_port_records_share_coords(fixture):
     """A port's Station record and Port record must hold the same coordinates.