diff --git a/CLAUDE.md b/CLAUDE.md
index c5c8515..821032c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -22,7 +22,7 @@ Vendors two submodules (see `.gitmodules`):
     cmake --build build
     ctest --test-dir build
 
-Six targets:
+Seven targets:
 
 | Target | Kind | Purpose |
 |---|---|---|
@@ -31,6 +31,7 @@ Six targets:
 | `capture_paths_tests` | executable | Pure unit tests for `capture_paths` — no REAPER, no DAW. |
 | `view_mode_model_tests` | executable | Pure unit tests for `view_mode_model` — no REAPER, no DAW. |
 | `view_tree_tests` | executable | Pure unit tests for `view_tree` — no REAPER, no DAW. |
+| `mode_switch_tests` | executable | Pure unit tests for `mode_switch` — no REAPER, no DAW. |
 | `reaper_reasampler` | loadable module | The actual extension binary (`.dll` / `.dylib` / `.so`). |
 
 ### macOS / Linux: SWELL dialog resources
@@ -52,6 +53,7 @@ There is no hot-reload. Copy the built binary into REAPER's `UserPlugins/` folde
 - `peaks` — waveform min/max bin computation from raw PCM. Fed a known signal, asserts envelope. Does not depend on REAPER's peak API.
 - `view_mode_model` — Design View mode system: mode registry, GUID-keyed membership, folder-tree-aware visibility derivation, snapshot-based park/restore planner, JSON round-trip. Mirror of `bank_model` for the Design View phase.
 - `view_tree` — pure `I_FOLDERDEPTH`→FolderTree helper for the Design View shell; no REAPER types at the boundary.
+- `mode_switch` — REAPER-free segment layout + hit-test math for the bank_panel's Design View mode switch; divides a header rectangle into N equal segments and hit-tests a point to a segment. Mirror of `bank_grid`.
 
 **REAPER-facing shells:**
 - `capture` — `ICaptureBackend` interface; `OfflineRenderBackend` (deterministic default) and `RealtimeRecordBackend`. Input: `CaptureRequest`. Output: finished file + populated `Sample` handed to `bank_model`.
diff --git a/COMPLETED.md b/COMPLETED.md
index db0f4c4..e796d2f 100644
--- a/COMPLETED.md
+++ b/COMPLETED.md
@@ -239,6 +239,28 @@ toggle + mode-jumps MIDI-bindable; tag/untag acts on the current track selection
 
 ---
 
+## D5 — in-window toggle affordance (UI)
+**Goal:** The segmented mode switch in the ReaSampler / bank_panel window header.
+CONTEXT.md §Design View (UI). **Verify (in DAW):** Segmented control shows current
+mode (lit segment), one click flips modes via the D4 toggle action, per-mode
+membership count visible, offlined-FX caveat surfaced as a tooltip.
+
+- [x] Segmented mode switch `[ Arrange | Design ]` in the window header; active lit.
+- [x] Wire the switch to the toggle/activate actions from D4.
+- [x] Per-mode membership count display.
+- [x] Offlined-FX re-init caveat as a tooltip on the switch.
+
+**Notes/decisions:**
+- New PURE module `src/mode_switch.{h,cpp}` — REAPER-free layout math for the
+  Design View mode switch: divides a header rectangle into N equal segments (one per
+  registered mode) and hit-tests a point to a segment. Unit-tested via a new
+  `mode_switch_tests` CTest target. Mirror of `bank_grid`.
+- `src/bank_panel.cpp` — segmented `[ Arrange | Design ]` control drawn in the panel
+  header (one lit segment per registered mode, click activates that mode via
+  `view::applyMode`), with the grid offset below the header.
+
+---
+
 ## Milestone 7 — capture action family
 **Goal:** Bindable capture actions for master / selected tracks / selected items /
 razor area, each with wet-dry + tail options. CONTEXT.md §actions, Build order 7.
diff --git a/PLAN.md b/PLAN.md
index b9b49e3..5d6561d 100644
--- a/PLAN.md
+++ b/PLAN.md
@@ -79,35 +79,3 @@ landed milestone.
   in int64 but exceed `INT_MAX` are implementation-defined. Hardening candidate
   — add bounds check before the cast when integer-field validation is in scope.
 
----
-
-# Phase D — Design View (parallel to the M0–M11 capture roadmap)
-
-> **Separate phase namespace.** The M-numbers belong to the capture pillar
-> (M0–M11, M4 in flight). Design View is an orthogonal feature on its own track,
-> so it uses a **lettered** namespace (D1, D2, …) rather than extending the M-line.
-> This keeps the two roadmaps from colliding on numbering and reads correctly:
-> Phase D is not "the twelfth capture step," it's a different pillar. Authoritative
-> spec: **CONTEXT.md §Design View**. Product framing: `docs/product/design-view.md`.
-> When a point lands, doc-keeper moves it to `COMPLETED.md`.
-
-## D5 — in-window toggle affordance (UI)
-**Goal:** The segmented mode switch in the ReaSampler / bank_panel window header.
-CONTEXT.md §Design View (UI). **Verify (in DAW):** Segmented control shows current
-mode (lit segment), one click flips modes via the D4 toggle action, per-mode
-membership count visible, offlined-FX caveat surfaced as a tooltip.
-
-- [ ] Segmented mode switch `[ Arrange | Design ]` in the window header; active lit.
-- [ ] Wire the switch to the toggle/activate actions from D4.
-- [ ] Per-mode membership count display.
-- [ ] Offlined-FX re-init caveat as a tooltip on the switch.
-
-## Phase D open questions
-- **Snapshot durability across save-while-parked** — persisted here by decision;
-  lean alternative is force-restore-to-Arrange on save (product notes item 4).
-  (touches D3)
-- **Reconcile on delete/restructure** — tagged leaf deleted or folder restructured
-  while parked leaves a stale GUID; ignore-and-prune on next toggle/open.
-  (touches D1/D2)
-- **Screenset coexistence** — Design View drives the same flags a screenset recall
-  would; confirm last-writer-wins is not surprising. (touches D2)