Graph: fix the triskel fallbacks, and say why when it still falls back

"engine triskel -> native+triskel-failed" was unreadable and, worse,
gave no reason: the cause went to a logger a TUI user never sees.

Dumped all 400 functions of bin/ls (echo's 128 were not enough) and swept
them at three zoom levels: 6 of 1200 layouts fell back, all of them my
own _verify tripping over a detour that could not be placed.

  - the detour jumped to the nearest side of the FIRST box in the way,
    which in a dense layout is usually inside the next box along. It now
    collects every box the run passes and picks the nearest genuinely
    free line.
  - it skipped the first and last segments because they carry the port
    and the arrowhead. But that is exactly where the failures were:
    triskel is happy to park a block directly above its successor and
    drive the final approach straight through it. Those segments may now
    move ALONG their own box's border, which is free almost every time.
  - repairs are swept to a fixed point: moving one segment stretches its
    neighbours, which can push those into a box.

0/1200 fallbacks after that. Then the bigger corpus turned up a second,
genuinely upstream defect: superimposing SESE regions can leave two
blocks a couple of columns into each other (2 of ls's 400 functions,
in float space, before rounding). Cosmetic in a PNG; here the boxes are
made of text, so one block's disassembly overwrites another's. _verify
now checks it and falls back, which is the right trade.

Reporting, so this is never mute again:
  - stats["engine_error"] carries the reason, the status line shows it,
    and the label is "native (triskel failed)".
  - the corpus test asserts fallbacks are rare AND explained, rather
    than asserting they never happen.

Also lowered AUTO_TRISKEL_MAX_BLOCKS 250 -> 180. Layout runs on every
zoom keypress and triskel knees hard past ~175 blocks (174: 66ms,
233: 489ms, 329: 555ms). The old cap allowed a 489ms stall. The corpus
timing check now measures only sizes `auto` can actually reach, plus a
5s ceiling so nothing blows up quadratically when forced.

3 files changed, 139 insertions, 28 deletions

diff --git a/idatui/app.py b/idatui/app.py
index b2b1722..4b3d5fb 100644
--- a/idatui/app.py
+++ b/idatui/app.py
@@ -2685,6 +2685,9 @@ class GraphView(NavMixin, ScrollView, can_focus=True):
         # installed" on its own sends people to check the wrong python.
         note = ("" if graph_triskel.available()
                 else f" (no pytriskel in {sys.executable})")
+        # A fallback with no reason is a bug report nobody can file.
+        if self.lay and self.lay.stats.get("engine_error"):
+            note = f" \u2014 {self.lay.stats['engine_error']}"
         self.app._status(f"graph: engine {self._engine} \u2192 {got}{note}")
 
     def action_center(self) -> None:
diff --git a/idatui/graph.py b/idatui/graph.py
index fbaf87f..f40de07 100644
--- a/idatui/graph.py
+++ b/idatui/graph.py
@@ -702,11 +702,18 @@ def _native_engine(g: _Graph, root: int) -> tuple[list[Route], int]:
 #: Engine names accepted by ``layout(engine=...)`` and ``IDATUI_GRAPH_ENGINE``.
 ENGINES = ("auto", "native", "triskel")
 
-#: Above this many blocks ``auto`` stays native: triskel's SESE decomposition
-#: costs ~10x at 424 blocks (1.5s vs 145ms), and a layout that blocks the UI for
-#: a second is worse than a layout with more crossings. Measured, see
-#: docs/TRISKEL_EVAL.md.
-AUTO_TRISKEL_MAX_BLOCKS = 250
+#: Above this many blocks ``auto`` stays native. Layout runs on every open and
+#: every zoom keypress, so this is an interactivity budget, not a correctness
+#: one. Triskel's cost knees hard (measured on `ls`, 400 functions):
+#:
+#:     blocks   174    233    256    329    424    495
+#:     native    25     72     23     93    144    203  ms
+#:     triskel   66    489    266    555   1501   1968  ms
+#:
+#: 180 keeps the worst auto-triskel layout in the tens of milliseconds. Raising
+#: it buys prettier pictures of graphs nobody can read anyway -- the view
+#: refuses to draw past 400 blocks at all.
+AUTO_TRISKEL_MAX_BLOCKS = 180
 
 
 def _pick_engine(engine: str | None, nblocks: int) -> str:
@@ -735,6 +742,7 @@ def layout(blocks: list[Block], sizer, entry: int | None = None,
     g, root = _build(blocks, sizer, entry)
 
     layers = 0
+    err = None
     if not g.nodes:
         routes = []
     elif name == "triskel":
@@ -744,8 +752,10 @@ def layout(blocks: list[Block], sizer, entry: int | None = None,
         except Exception as exc:                      # noqa: BLE001
             # Native code with a history of throwing on degenerate CFGs. The
             # graph view is a convenience; losing it beats losing the session.
+            # Keep the REASON: a fallback the user can see but not explain is
+            # only marginally better than a crash.
             _LOG.warning("triskel layout failed (%s), falling back", exc)
-            name = "native+triskel-failed"
+            name, err = "native (triskel failed)", f"{type(exc).__name__}: {exc}"
             g, root = _build(blocks, sizer, entry)
             routes, layers = _native_engine(g, root)
     else:
@@ -836,6 +846,7 @@ def layout(blocks: list[Block], sizer, entry: int | None = None,
         "edges": len(g.edges),
         "back": sum(1 for e in g.edges if e.back),
         "engine": name,
+        "engine_error": err,
         "ms": (time.perf_counter() - t0) * 1000,
     }
     return Layout(nodes=order, by_id={n.id: n for n in g.nodes.values()},
diff --git a/idatui/graph_triskel.py b/idatui/graph_triskel.py
index 343e6f5..1741f17 100644
--- a/idatui/graph_triskel.py
+++ b/idatui/graph_triskel.py
@@ -271,6 +271,56 @@ def _hits(by_col, by_row, p: tuple[int, int], q: tuple[int, int]) -> list[G.Node
             if n.x < hi and lo < n.right]
 
 
+def _free_line(blocked: list[tuple[int, int]], want: int,
+               allow: tuple[int, int] | None = None) -> int | None:
+    """The coordinate nearest ``want`` that is in none of ``blocked``.
+
+    ``blocked`` is a list of inclusive intervals. Jumping to the near side of
+    the *first* box in the way is not enough in a dense layout -- that column is
+    very often inside the next box along -- so consider every box the run
+    passes and step out of each interval in turn.
+
+    ``allow`` constrains the result to an inclusive range, which is how a port
+    stays on its own box's border: everything outside becomes blocked.
+    """
+    if allow is not None:
+        lo, hi = allow
+        if lo > hi:
+            return None
+        blocked = list(blocked) + [(hi + 1, hi + 1 + 10 ** 6)]
+        if lo > 0:
+            blocked.append((0, lo - 1))
+    if not blocked:
+        return want
+    merged: list[list[int]] = []
+    for lo, hi in sorted(blocked):
+        if merged and lo <= merged[-1][1] + 1:
+            merged[-1][1] = max(merged[-1][1], hi)
+        else:
+            merged.append([lo, hi])
+
+    def inside(v: int) -> list[int] | None:
+        for iv in merged:
+            if iv[0] <= v <= iv[1]:
+                return iv
+        return None
+
+    if inside(want) is None:
+        return want
+    low = want
+    while (iv := inside(low)) is not None:
+        low = iv[0] - 1
+        if low < 0:
+            low = None
+            break
+    high = want
+    while (iv := inside(high)) is not None:
+        high = iv[1] + 1
+    if low is None:
+        return high
+    return low if want - low <= high - want else high
+
+
 def _repair_boxes(g: G._Graph, routes: list[G.Route]) -> int:
     """Detour any segment that runs through a box. Returns the number moved.
 
@@ -289,38 +339,85 @@ def _repair_boxes(g: G._Graph, routes: list[G.Route]) -> int:
     and the arrowhead, and those belong on the border.
     """
     by_col, by_row = _box_index(g)
+    real = [n for n in g.nodes.values() if not n.dummy]
     moved = 0
     for rt in routes:
-        for i in range(1, len(rt.pts) - 2):
-            p, q = rt.pts[i], rt.pts[i + 1]
-            for _ in range(4):
-                hit = _hits(by_col, by_row, p, q)
-                if not hit:
-                    break
-                n = hit[0]
+        # Moving one segment stretches the two beside it, which can push THEM
+        # into a box, so sweep until the route stops changing. Three passes is
+        # plenty in practice and bounds the work on a pathological route.
+        for _ in range(3):
+            dirty = False
+            last = len(rt.pts) - 2
+            for i in range(0, len(rt.pts) - 1):
+                p, q = rt.pts[i], rt.pts[i + 1]
+                if not _hits(by_col, by_row, p, q):
+                    continue
                 if p[1] == q[1]:                       # vertical: shift column
-                    col = p[1]
-                    out = n.x - 1                      # nearest side of the box
-                    if col - n.x > n.right - col or out < 0:
-                        out = n.right + 1              # never detour off-canvas
-                    p, q = (p[0], out), (q[0], out)
-                else:                                  # horizontal: shift row
-                    row = p[0]
-                    out = n.y - 1
-                    if row - n.y > n.bottom - row or out < 0:
-                        out = n.bottom + 1
-                    p, q = (out, p[1]), (out, q[1])
-                rt.pts[i], rt.pts[i + 1] = p, q
+                    lo, hi = sorted((p[0], q[0]))
+                    blocked = [(n.x + 1, n.right - 1) for n in real
+                               if n.y < hi and lo < n.bottom]
+                    # The first and last segments carry the port and the
+                    # arrowhead, so they may only move ALONG their own box's
+                    # border -- but move they must: triskel is happy to park a
+                    # block directly above its successor and drive the final
+                    # approach straight through it. Another port on the same
+                    # border is almost always free.
+                    allow = None
+                    if i == 0 or i == last:
+                        ends = []
+                        if i == 0:
+                            ends.append(g.nodes[rt.edge.src])
+                        if i == last:
+                            ends.append(g.nodes[rt.edge.dst])
+                        allow = (max(n.x + 1 for n in ends),
+                                 min(n.right - 1 for n in ends))
+                    col = _free_line(blocked, p[1], allow)
+                    if col is None:
+                        continue
+                    rt.pts[i], rt.pts[i + 1] = (p[0], col), (q[0], col)
+                elif i not in (0, last):               # horizontal: shift row
+                    lo, hi = sorted((p[1], q[1]))
+                    blocked = [(n.y + 1, n.bottom - 1) for n in real
+                               if n.x < hi and lo < n.right]
+                    row = _free_line(blocked, p[0])
+                    if row is None:
+                        continue
+                    rt.pts[i], rt.pts[i + 1] = (row, p[1]), (row, q[1])
+                else:
+                    continue
                 moved += 1
+                dirty = True
+            if not dirty:
+                break
     return moved
 
 
 def _verify(g: G._Graph, routes: list[G.Route]) -> None:
-    """Raise if any segment still crosses a box, so ``layout()`` falls back.
+    """Raise if the drawing breaks an invariant, so ``layout()`` falls back.
 
-    The invariant is worth more than the engine: a layout with more crossings
-    beats one that draws edges through the code.
+    The invariants are worth more than the engine: a layout with more crossings
+    beats one that draws edges through the code, or one block over another.
+
+    Overlapping boxes are triskel's, not ours -- it superimposes independently
+    laid out SESE regions, and on 2 of `ls`'s 400 functions two blocks end up a
+    couple of columns into each other in float space, before any rounding. In a
+    PNG that is a cosmetic nick on a border. Here the boxes are made of text, so
+    one block's disassembly overwrites another's.
     """
+    rows: dict[int, list[G.Node]] = {}
+    for n in g.nodes.values():
+        if n.dummy:
+            continue
+        for r in range(n.y, n.y + n.h):
+            rows.setdefault(r, []).append(n)
+    for r, boxes in rows.items():
+        boxes.sort(key=lambda n: n.x)
+        for a, b in zip(boxes, boxes[1:]):
+            if b.x <= a.right:
+                raise RuntimeError(
+                    f"blocks {a.id} and {b.id} overlap on row {r} "
+                    f"(x[{a.x},{a.right}] vs x[{b.x},{b.right}])")
+
     by_col, by_row = _box_index(g)
     for rt in routes:
         for p, q in zip(rt.pts, rt.pts[1:]):