@@ -9,7 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Added
 
-- **ID correspondence between views (3-D projects).** A new **Views** page (page 10, under the Sessions row of the sidebar, reached by Next/Back between Sessions and Calibration; 3-D projects only) records which session is the top view and which the side view, pairs them by two session-name patterns with a `key` group (with an ⓘ help and a live "catches N sessions" line) or by hand, and maps each top-view fish to a side-view fish: by a "Same IDs" tick, or in a table with two highlighted track plots. The project file gains `SessionRef.view_role`, `view_pairs` and `mode.pairing`; `mode.id_map` is gone (projects that contain it still load). It never blocks Next, and 3-D projects still cannot be run.
+- **Fusion of top and side views (3-D projects).** The Views page gains a **Fusion** section for the selected pair with a status line and **Set up fusion…**. The dialog shows the side view with two draggable lines for the water surface and the tank floor, fields for both rows, the tank height (default 20 cm), the shared horizontal axis (top-view x or y) with Flip, and the frame offset (hidden and fixed at 0 for "One video, two panels"), a live summary (shared frames, fused fish, positions outside the water column, agreement) and **Suggest offset**, which scans ±5 s around 0 (about 3 s on a 1-hour, 5-fish recording; needs a calibrated top view, otherwise it is disabled with a hint). The fusion keeps the fish matched in the pair on the frames both views have and adds a depth (0 = surface, 1 = floor; positions outside the column become empty and are counted). The cameras are assumed to be at right angles; the two views' horizontal positions are compared in cm and a warning shows when they disagree by more than 10% of the top-view range (needs a calibrated top view). Frame rates must match. New `FusionSettings` on each pair (`ViewPair.fusion`), `PreprocessedSession.depth`, `Engine.fuse_pair`, `fuse_all` and `suggest_offset`. The Views status warns "Fusion setup needed for …" and never blocks. 3-D projects still cannot be run: the 3-D metrics step is not built.
+- **Note on provenance hashes:** `project_hash` now includes each session's (empty) `panel` field, so the provenance hash of an existing project changes once after this update. Results are not affected.
+- **Panels: one video, two views (3-D projects, "One video, two panels" layout).** A session can now have a *panel*, the rectangle of the video it covers (`SessionRef.panel`; projects without it load unchanged). Reading a session with a panel keeps the animals that have at least 50% of their positions inside it, sets positions outside to NaN and reports coordinates relative to the panel (its top-left is (0, 0), the video size becomes the panel size), so previews, runs and the CLI all see the panel. The same folder with a different panel is a different session, and the panel is part of the preprocessing cache key. The Views page gains a **Panels** section (that layout only) with **Split into panels…** (one tracker run on the whole frame becomes `<id>__top` and `<id>__side` plus a hand-made pair), **Set panel…** (two tracker runs on one video, each limited to one panel) and **Clear panel**. The panel editor has the presets Left | Right and Top | Bottom with a split slider, exact x, y, width and height fields, a preview over the tracker's background image (or the first video frame, or the tracks), and a per-fish table that flags fish under 90% inside as *low* and under 50% as *left out*. Changing or clearing a panel resets the fish map of the pair that holds the session. The zones backdrop, calibration and preview show the cropped panel. Zones are still one set per project, and per-view pixel-scale calibration is not part of this change. 3-D projects still cannot be run.
+- **ID correspondence between views (3-D projects).** A new **Views** page (page 10, under the Sessions row of the sidebar, reached by Next/Back between Sessions and Calibration; 3-D projects only) records which session is the top view and which the side view, pairs them by two session-name patterns with a `key` group (with an ⓘ help and a live "catches N sessions" line) or by hand, and maps each top-view fish to a side-view fish: by a "Same IDs" tick, or in a table with two highlighted track plots. The project file gains `SessionRef.view_role`, `view_pairs` and `mode.pairing`; `mode.id_map` is gone (projects that contain it still load). The fish map is what counts: a pair is matched when its map is not empty. **Pair by pattern** keeps the pairs it made earlier that the patterns still produce, with their matching, removes the ones they no longer produce, and skips pairs you made or changed by hand (ticking Same IDs or changing a fish makes a pair yours). In 3-D projects the Sessions row of the sidebar shows the worse of the Sessions and Views statuses, with the Views message in its tooltip. It never blocks Next, and 3-D projects still cannot be run.
 - **Session calibration works beyond idtracker.ai.** The Calibration screen describes the mode per tracker, with an ⓘ note linking to each tracker's documentation, and disables it when no session can supply a scale. DeepLabCut, SLEAP and Ctrax raw files take an optional **Scale (pixels per cm)** import option.
 - **`trx.mat` reader (Ctrax / FlyTracker / JAABA).** Built from the documented layout, unverified against a real sample. `pxpermm` becomes the session scale.
 - **Average or median of several idtracker.ai length calibrations.** The average (idtracker.ai's own `length_unit`) stays the default.
 
@@ -214,6 +214,7 @@ def __init__(self, parent: QWidget | None = None) -> None:
             self._store.exportChanged,
             self._store.runResultsChanged,
             self._store.modeChanged,
+            self._store.viewsChanged,
         ):
             sig.connect(self._refresh_stage_status)
         self._store.runLogAppended.connect(self._run_log.append)
@@ -507,7 +508,7 @@ def _on_project_opened(self) -> None:
 
     def _refresh_stage_status(self) -> None:
         from app.navigation import STAGES
-        from ui.store.stage_status import compute_stage_statuses, stage_summaries
+        from ui.store.stage_status import compute_stage_statuses, sessions_row, stage_summaries
 
         self._page_statuses = compute_stage_statuses(
             self._store.manifest, has_run_results=self._store.run_results is not None
@@ -528,6 +529,8 @@ def _refresh_stage_status(self) -> None:
                 summaries[7] = summaries[8] = f"{len(results) - failed} succeeded · {failed} failed"
         for stage_index, (_label, first_page) in enumerate(STAGES):
             info = self._page_statuses[first_page]
+            if stage_index == 1:
+                info = sessions_row(self._page_statuses, manifest)
             self._sidebar.set_status(
                 stage_index, info.status, info.message, summary=summaries[stage_index]
             )
@@ -555,7 +558,7 @@ def _update_next_action(self) -> None:
         self._refresh_footer_labels(page)
 
     def _refresh_footer_labels(self, page: int) -> None:
-        from app.navigation import PAGE_TO_STAGE, STAGES
+        from app.navigation import PAGE_TO_STAGE, STAGES, page_label
 
         mode = self._mode()
         prev_p, next_p = prev_page(mode, page), next_page(mode, page)
@@ -569,7 +572,7 @@ def _refresh_footer_labels(self, page: int) -> None:
         elif next_p is None:
             text, role = "Done", "primary"
         else:
-            text = f"Next: {STAGES[PAGE_TO_STAGE[next_p]][0]} →".replace("&", "&&")
+            text = f"Next: {page_label(next_p)} →".replace("&", "&&")
             role = "primary"
         self._btn_next.setText(text)
         if self._btn_next.property("role") != role:
 
@@ -12,6 +12,8 @@
 )
 
 from app.theme import theme
+from ui.store.screen_flow import VIEWS_PAGE
+from ui.store.stage_status import VIEWS_PAGE_NAME
 
 # 9 wizard stages, one sidebar row per page (Preview and Export share the
 # last row). The number lives in the painted dot, not the label.
@@ -28,14 +30,19 @@
     ("Preview & Export", 8),
 ]
 
-# The 3-D-only Views page: appended after the original 10, so their indices
-# are unchanged. It is routed between Sessions and Calibration (screen_flow).
-VIEWS_PAGE = 10
+# The 3-D-only Views page (VIEWS_PAGE, defined in ui.store.screen_flow) is
+# appended after the original 10, so their indices are unchanged. It is routed
+# between Sessions and Calibration (screen_flow).
 
 # Maps each page index → its parent stage index (9 stages, 11 pages; Views
 # sits under Sessions).
 PAGE_TO_STAGE: list[int] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 8, 1]
 
+
+def page_label(page: int) -> str:
+    """The name the footer uses for *page*: its stage, or "Views" for the Views page."""
+    return VIEWS_PAGE_NAME if page == VIEWS_PAGE else STAGES[PAGE_TO_STAGE[page]][0]
+
 STATUS_ROLE = Qt.ItemDataRole.UserRole
 SUMMARY_ROLE = Qt.ItemDataRole.UserRole + 1
 LOCKED_ROLE = Qt.ItemDataRole.UserRole + 2
 
@@ -1,6 +1,6 @@
 # 3-D movement: roadmap
 
-**Status:** approved 2026-10-08. Sub-project C is in progress; E (mode switch) is implemented; A, B and D are not started.
+**Status:** approved 2026-10-08. Sub-project C is in progress; E (mode switch), G, F and D (fusion) are implemented; A and B are not started.
 **Relates to:** `docs/dev/DECISIONS.md` D-031, `docs/dev/ROADMAP.md` (Z-10),
 `docs/tracker-formats/2026-10-07-tracker-import-design.md` (Tier 3)
 
@@ -11,14 +11,15 @@ build cycle.
 |---|---|---|
 | **C** | **Side-view depth**: a camera-view setting and a continuous vertical-position metric from ordinary 2-D side recordings | nothing unmerged, no 3-D data |
 | **E** | **2D/3D project mode**: `ProjectManifest.mode`, Project-screen choice, lock, 3-D compute blocked. Done; see [the spec](2026-10-09-mode-switch-design.md) | nothing |
+| F | **Panel split**: one video with the top and side views in two panels becomes two sessions (a panel rectangle per session, panel-relative coordinates). Done; see [the spec](2026-10-09-panel-split-design.md) | E |
 | G | **ID correspondence**: view roles, pairing top and side sessions (regex or by hand), fish mapping between views, a Views step. Done; see [the spec](2026-10-09-id-correspondence-design.md) | E |
 | A | Read 3-D tracker files (Anipose pose-3d CSV, DLC-3D). Metrics run on a chosen plane and z is stored (D-031 as written) | `feat/native-units` merged |
 | B | Metrics that use z: 3-D speed and path length, vertical position, 3-D neighbour distance. **Amends D-031** | A for data, C for the vertical-axis setting |
-| D | Fuse top and side recordings into one 3-D session | B's data model, frame sync, identity matching across views |
+| D | Fuse top and side recordings into one 3-D session. Done; see [the spec](2026-10-09-fusion-design.md) | F and G (done); compute stays blocked until B |
 
-**Order: C, A, B, D.** A moves first if real 3-D files turn up. C goes first because no data
-of any kind exists yet, it is testable on synthetic tracks, and it creates the project-level
-camera-view setting that B reuses.
+**Order: C, F/G/E and D are done; B then A remain.** A moves first if real 3-D files turn up. C went
+first because no data of any kind existed, it is testable on synthetic tracks, and it created the
+project-level camera-view setting that B reuses.
 
 ## Hand-offs between the sub-projects