Collapse a Design View mode switch and its background FX parking to ONE undo point

applyMode now returns with its undo block open; the reclaimer closes it when the drain
settles. A transport deferral or a tab switch ends it early -- two points, never a leak.

src/app/main.cpp

@@ -324,6 +324,10 @@ extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(
             // altered transport/cursor in the user's project on unload.
             capture::AbortRealtimeCaptureForUnload(g_session);
 
+            // Same reason, for the reclaimer's undo span: a block left open here has no
+            // owner left to close it and would swallow every edit the user makes after.
+            reasampler::fxReclaimEndUndoSpan();
+
             g_rec->Register("-timer", (void*)&OnTimer);
             g_rec->Register("-projectconfig", (void*)&g_projectConfig);
             g_rec->Register("-toggleaction", (void*)&OnToggleAction);

src/shell/view/CLAUDE.md

@@ -64,17 +64,35 @@ decide membership or mode rules.
   offline/online runs on the background reclaimer (`fx_reclaim`), in ticks
   bounded by `kOnlineBudgetMs`/`kOfflineBudgetMs`, off `applyMode`'s block
   entirely (measured: `TrackFX_SetOffline` was 1897ms of a 1942ms switch).
-- **ONE mode switch is ONE undo point.** The driven flags and the lane state
-  written inside `applyMode`'s block roll back in a single Ctrl-Z. (The
-  `view_state` ext-state write is NOT among them; it lands after the block, see
-  Gotchas.) Per-FX offline/online moved off this block entirely, onto the
-  reclaimer (`fx_reclaim`) — each write there runs inside its OWN discarded
-  block (`Undo_EndBlock2(proj, "", 0)`), so it mints nothing. DAW-VERIFIED: ten
-  unblocked `TrackFX_SetOffline` calls minted ten undo points; the same ten
-  calls wrapped in `Undo_BeginBlock2`/`Undo_EndBlock2(proj, "", 0)` minted zero
-  — confirming the discard-block premise the reclaimer's per-write block
-  (`fx_reclaim.cpp`'s `writeOffline`) rests on.
-- **A reapply that wrote nothing leaves no undo point.** The load-tick reapply
+- **ONE mode switch is ONE undo point — the background FX parking included.** The
+  driven flags, the lane state, AND the per-FX offline/online the reclaimer writes
+  minutes-of-plugin-loads later all roll back in a single Ctrl-Z, because they share
+  ONE undo block: `applyMode` opens it (`fx_reclaim`'s `UndoSpan`) and a REAL switch
+  RETURNS WITH IT OPEN, for the reclaimer to close once the drain has converged. A
+  reapply closes its own before returning. This does not move an FX write back onto
+  the switch — the writes still run on the timer, off the 1897ms path.
+  DAW-MEASURED, and the reason the design is shaped this way: block WIDTH is the
+  entire effect. For ten `TrackFX_SetOffline` calls from a TIMER — unblocked, or
+  under a block per write, by every dispatch route — REAPER minted TEN points; under
+  ONE block spanning all ten it minted ONE. The `("", 0)` discard form does NOT
+  discard outside an action, it coalesces; it discards only from a hand-fired action.
+  A block opened on one timer callback and closed on a LATER one holds. Whether one
+  held from an ACTION into a later timer callback holds is the same property but is
+  `[verify — DAW]`; if it does not, the switch and its parking land as two points,
+  which is the intended degradation.
+- **No path may return past a terminating condition with the span open.** A leaked
+  block swallows every edit the user makes afterwards, indefinitely, so every
+  terminating path routes through ONE idempotent close (`fxReclaimEndUndoSpan`): the
+  drain settling, aborting on a generation bump, or deferring on the transport; the
+  project retargeting or closing; the next apply superseding it; a project save;
+  extension unload. The span may stay open only across a wait it can BOUND — the 3s
+  debounce, which is the dead time between the switch and its first FX write, and
+  the only reason the span exists. A TRANSPORT deferral is not bounded (it lasts as
+  long as the playback pass), so it ends the span and the parking that follows mints
+  its own point (`spanEndsAfter`).
+- **A reapply that wrote nothing leaves no undo point** — and a reapply never holds
+  the span open, so a project open, a membership edit and `render_in_place`'s inner
+  reapply all keep the block shape they had. The load-tick reapply
   re-plans a park for every inactive leaf, and over a project saved fully parked
   it finds every flag and every FX already where the plan wants them. The block
   then closes on the `("", 0)` discard form (`view.h`'s `applyMintsUndoPoint`;
@@ -124,13 +142,13 @@ applies the resulting lane state to live tracks.
 
 ## Modules
 
-- `view` — Design View shell: runs the pure planner over the live folder tree and iterates its park/restore plans, applies managed-lane and parent-visibility writes, and owns the ONE undo block all of that rides (`applyMintsUndoPoint` decides whether it leaves a point). Owns the one discriminator (`target != active`) that separates a real switch from a reapply, and with it both the playback gate (`transportBlocksModeSwitch`) and the solo cache/clear/restore seams. **Never touches master or `B_MUTE`.** FX are NOT written here — see `fx_reclaim`.
+- `view` — Design View shell: runs the pure planner over the live folder tree and iterates its park/restore plans, applies managed-lane and parent-visibility writes, and OPENS the ONE undo span all of that rides (`applyMintsUndoPoint` decides whether it leaves a point; on a real switch `fx_reclaim` is what closes it). Owns the one discriminator (`target != active`) that separates a real switch from a reapply, and with it both the playback gate (`transportBlocksModeSwitch`) and the solo cache/clear/restore seams. **Never touches master or `B_MUTE`.** FX are NOT written here — see `fx_reclaim`.
 - `view_park` — the park surface, FLAG DOMAIN ONLY. `parkTrack` captures the pre-park snapshot (flags AND the FX chain's identity-keyed offline state) when the model holds none, drives the four flags, and EXPOSEs a track that reads park-shaped with no record behind it rather than adopting it; `restoreTrack` writes the flags back from the snapshot and spends its flag half (`releaseSnapshotFlags`). Also `trackFlagParm` (the one `Flag` → REAPER-parameter mapping), `writeIfChanged` (the read-before-write every driven parameter goes through), `fxGuidString` (the one FX-identity read snapshot/reclaim both address FX through), `kViewUndoMask`, and `reportExposedTracks`/`describeExposedTracks` (named tracks, once per changed set per project).
 - `view_lanes` — managed fixed lanes: `mintManagedLanes` splits multi-mode content onto per-mode lanes, `applyLaneOps` drives `C_LANEPLAYS` for the active mode, `reconcileManagedLanes` recovers ownership from durable `P_LANENAME` on project open. Manual lanes are never minted, driven, renamed or reassigned.
 - `view_reconcile` — the repair pass for a project state REAPER reloaded under us: `noteAppliedState` records what an apply left driven; `reconcileAppliedFlags` diffs that fingerprint against the reloaded model and hands back any track the reload no longer records but still reads driven — the fix for `UNDO_STATE_TRACKCFG` not carrying `B_SHOWINTCP`/`B_SHOWINMIXER` (DAW-confirmed).
 - `view_tracks` — the scaffolding every write pass sets up: `TrackHandles`/`TrackByGuid`, `readFolderEntries`/`indexHandles`, and `UiRefreshHold` (the `PreventUIRefresh` RAII pair every write phase runs under).
 - `view_solo` — the `I_SOLO` read/write pair behind the per-mode solo surface, plus `clearTrackSolos`/`restoreTrackSolos`, the outgoing-clear and incoming-replay entry points `view` drives them through. Holds no policy: what to cache, clear, or replay is `core/view/solo_cache`.
-- `fx_reclaim` — the FX convergence loop, off the switch's synchronous path (`TrackFX_SetOffline` was 1897ms of a measured 1942ms switch). `drainTick` moves one track's FX per step against `core/view/fx_state`'s derived work list (Online before Offline; an Offline item waits out `kParkHoldMs` so a fast A→B→A flip moves nothing), budgeted per tick (`kOnlineBudgetMs`/`kOfflineBudgetMs`) and gated off the transport for the Offline direction only. `FxReclaimQueue` owns the derived queue's lifetime across re-entrancy (a plugin unload is assumed to pump); each `writeOffline` runs inside its own discarded undo block, DAW-verified to mint nothing. `ReleaseFlush`/`fxReclaimFlushRelease` make a completed record's spend durable on the project's next save, never on the drain's own initiative.
+- `fx_reclaim` — the FX convergence loop, off the switch's synchronous path (`TrackFX_SetOffline` was 1897ms of a measured 1942ms switch). `drainTick` moves one track's FX per step against `core/view/fx_state`'s derived work list (Online before Offline; an Offline item waits out `kParkHoldMs` so a fast A→B→A flip moves nothing), budgeted per tick (`kOnlineBudgetMs`/`kOfflineBudgetMs`) and gated off the transport for the Offline direction only. `FxReclaimQueue` owns the derived queue's lifetime across re-entrancy (a plugin unload is assumed to pump). `UndoSpan` owns the ONE undo block the whole drain writes under — opened by `applyMode` on a real switch, else lazily by the first `writeOffline`, and closed through the single idempotent `fxReclaimEndUndoSpan` that every terminating path routes through (`spanEndsAfter` decides which ticks terminate). `ReleaseFlush`/`fxReclaimFlushRelease` make a completed record's spend durable on the project's next save, never on the drain's own initiative.
 
 ## Gotchas
 
@@ -158,15 +176,24 @@ applies the resulting lane state to live tracks.
   reads identically and is EXPOSEd too — no lost state there at all, which is
   why the report never asserts a cause. A REDO of a mode switch is a further,
   hypothetical route — see the `view_state` gotcha below.
-- **An UNDO of a mode switch no longer rolls back FX at all.** FX offline/online
-  moved off `applyMode`'s block entirely onto the reclaimer, so a Ctrl-Z restores
-  only the flags and lane state the block wrote — whatever the reclaimer had
-  converged (or had not yet converged) survives the undo untouched. Whether this
-  reads as correct in the DAW is `[verify — DAW]`, `docs/VERIFICATION.md`
-  §"Mode switching".
-- **The `view_state` write is NOT inside `applyMode`'s block.** `persistViewState`
-  (`design_view_actions.cpp`) runs after `applyMode` has returned, so
-  `SetProjExtState` lands past `Undo_EndBlock2`. The UNDO direction still comes
+- **An UNDO of a mode switch rolls back the FX the span covered, and only those.**
+  The span carries `UNDO_STATE_FX` only once the drain has actually written FX under
+  it (`UndoSpan::widen`), so a switch whose parking never matured — a fast A→B→A
+  flip — still closes on `kViewUndoMask` alone and costs no FX serialization. What a
+  Ctrl-Z cannot restore is anything the reclaimer converges AFTER the span closed: a
+  transport deferral, and a drain no switch opened a span for, each mint their own
+  point. Whether that reads as correct in the DAW is `[verify — DAW]`,
+  `docs/VERIFICATION.md` §"Mode switching".
+- **The `view_state` write is NOT inside `applyMode`'s block — but on a REAL switch
+  it is now inside the SPAN.** `persistViewState` (`design_view_actions.cpp`) runs
+  after `applyMode` has returned, and a real switch returns with the span open, so
+  the `SetProjExtState` — and the modal Save-As `persistViewState` can pop on an
+  unsaved project — land inside the block after all. That is a consequence of the
+  span, not a decision made here; it does not resolve the redo hypothesis below,
+  which turns on capture-time semantics the SDK does not document. `[verify — DAW]`
+  that a Save-As dialog opening under an open undo block is harmless. The rest of
+  this gotcha describes the reapply path, where nothing changed. The UNDO direction
+  still comes
   out right — the point closed over the ext state as it stood BEFORE the switch,
   which is what a Ctrl-Z should restore — but by position, not by design. The
   REDO direction is what this leaves open, and the cost is worse than the

src/shell/view/fx_reclaim.cpp

@@ -38,6 +38,28 @@ FxReclaimQueue g_queue;
 // A release applied to the in-memory model and not yet written — see ReleaseFlush.
 ReleaseFlush g_releaseFlush;
 
+// The one open undo block a mode switch and its parking share — see UndoSpan.
+UndoSpan g_span;
+
+// The ONE open and the ONE close. Both idempotent, so every caller may say either
+// unconditionally; the close reads the span's values before clearing it, because the
+// clear has to precede the SDK call — nothing re-entered through Undo_EndBlock2 may
+// find a span it could close a second time.
+void openSpan(ReaProject* proj, const char* label, int mask) {
+    if (g_span.begin(proj, label, mask)) Undo_BeginBlock2(proj);
+}
+
+void closeSpan() {
+    if (!g_span.open()) return;
+    ReaProject* const proj  = g_span.project();
+    const std::string label = g_span.label();
+    const int         mask  = g_span.mask();
+    g_span.end();
+    // The span outlives its opener, so by here the project may be the one that just
+    // closed — its undo history went with it and there is no block left to end.
+    if (ValidatePtr2(nullptr, proj, "ReaProject*")) Undo_EndBlock2(proj, label.c_str(), mask);
+}
+
 // TrackFX_SetOffline is ASSUMED to pump the message loop, so a plugin load can re-enter
 // the timer and land back here mid-drain. Fail closed: a re-entered drain would derive a
 // second queue for the same project and race the outer one's writes track for track.
@@ -132,16 +154,15 @@ struct ReclaimHost {
         if (fxIndex < 0 || fxIndex >= TrackFX_GetCount(tr)) return false;  // chain shrank
         if (TrackFX_GetOffline(tr, fxIndex) == offline) return false;      // already there
 
-        // DAW-VERIFIED: the same ten TrackFX_SetOffline calls mint ten undo points
-        // unblocked and ZERO inside this pair. One block per WRITE even though a step is
-        // now one whole track: the write itself is what pumps, so the block cannot avoid
-        // spanning a pump — it can only span as little of one as possible. Widening it to
-        // the track would multiply that exposure by the chain length while suppressing no
-        // extra undo point (the discard form mints none either way), and a user action
-        // landing inside a discarded block can lose its own Ctrl-Z.
-        Undo_BeginBlock2(project);
+        // Every write in the drain rides ONE span. Do not reintroduce a block per write:
+        // it minted one undo point per FX toggle, because the ("", 0) discard form
+        // coalesces rather than discards outside an action (see UndoSpan). The open is a
+        // no-op under a switch's span, which is why the FX bit is added separately —
+        // here, at the first real write, so a switch whose parking never matured closes
+        // on the flags-and-items mask it opened with and serializes no FX state.
+        openSpan(project, kReclaimUndoLabel, UNDO_STATE_FX);
+        g_span.widen(UNDO_STATE_FX);
         TrackFX_SetOffline(tr, fxIndex, offline);
-        Undo_EndBlock2(project, "", 0);
         return true;
     }
 
@@ -188,6 +209,8 @@ void invalidateFxReclaim() {
 bool fxReclaimDrainInProgress() { return g_draining; }
 
 void fxReclaimTick(ReaSamplerSession& session, ReaProject* proj) {
+    // Neither guard may close the span: both name a caller that is still on the stack and
+    // still owns it — the outer drain, or the applyMode that opened it. Both re-tick.
     if (g_draining) return;
     // The model is mid-rewrite and its records are being consumed out from under any
     // plan derived from it. The apply invalidates on its way out, so nothing is lost.
@@ -200,6 +223,12 @@ void fxReclaimTick(ReaSamplerSession& session, ReaProject* proj) {
     // that gate that poll.
     ReaProject* project = proj ? proj : EnumProjects(-1, nullptr, 0);
     if (!ValidatePtr2(nullptr, project, "ReaProject*")) project = nullptr;
+
+    // A span belongs to the project it was opened on. Keyed on the SPAN's own project
+    // rather than the queue's pin, which moves independently: a tab switch, a closed
+    // project, or no active project at all ends it where it stands rather than carrying
+    // it into another project's history — and the no-project arm below returns.
+    if (g_span.open() && g_span.project() != project) closeSpan();
     if (!g_queue.beginTick(project)) return;
 
     // DERIVED, never persisted: recomputed from view state every time the queue was
@@ -213,9 +242,20 @@ void fxReclaimTick(ReaSamplerSession& session, ReaProject* proj) {
     ReclaimHost host{project, &session};
     const ReclaimRunResult run = g_queue.run(host);
     reportDrops(project, run.drops);
+    if (spanEndsAfter(run)) closeSpan();
     if (!run.aborted && run.settled) g_queue.goIdle();
 }
 
+void fxReclaimBeginUndoSpan(ReaProject* proj) {
+    openSpan(proj, "", 0);
+}
+
+void fxReclaimLabelUndoSpan(const char* label, int mask) {
+    g_span.relabel(label ? label : "", mask);
+}
+
+void fxReclaimEndUndoSpan() { closeSpan(); }
+
 void fxReclaimFlushRelease(ReaSamplerSession& session) {
     // GetCurrentProjectInLoadSave() names the project THIS hook is being asked to save
     // (reaper_plugin_functions.h:1582-1587) -- not merely the active tab, which a
@@ -224,6 +264,9 @@ void fxReclaimFlushRelease(ReaSamplerSession& session) {
     // the project actually being written, or match the arm against the active tab while a
     // different project saved and write into that wrong project's ext state.
     ReaProject* saving = GetCurrentProjectInLoadSave();
+    // BEFORE the arm test, and unconditionally: a save is a user act the span must not
+    // absorb, and the arm is not what decides whether one is open.
+    closeSpan();
     if (!g_releaseFlush.pendingFor(saving)) return;
     // Promptless, unlike design_view_actions' persistViewState, which can open a modal
     // Save-As. [verify — DAW] whether this write reaches the <EXTSTATE> block of the

src/shell/view/fx_reclaim.h

@@ -7,6 +7,7 @@
 #include <cstddef>
 #include <map>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "core/view/fx_state.h"
@@ -155,6 +156,12 @@ struct ReclaimTickResult {
     int  writes  = 0;
     bool settled = false;
 
+    // The transport gate — and only it — held an otherwise-eligible item back this
+    // tick. Distinguished from the debounce because the two waits differ in KIND: a
+    // hold expires at kParkHoldMs, a deferral lasts as long as the playback pass does,
+    // and the undo span may only stay open across a wait it can bound (spanEndsAfter).
+    bool transportDeferred = false;
+
     // Tracks whose park record the tick drove fully online, so the record's FX half is
     // spent. REPORTED, not applied: releasing is a model write and the model is exactly
     // what a re-entered load can replace mid-drain, so only the owner may commit it —
@@ -178,8 +185,9 @@ struct ReclaimTickResult {
 //                                                        may still online while it rolls
 //   bool   liveChain(guid, std::vector<std::string>&)    FX identity per live slot;
 //                                                        false when the track is gone
-//   bool   writeOffline(guid, fxIndex, offline)          compare-before-write inside its
-//                                                        own discarded undo block;
+//   bool   writeOffline(guid, fxIndex, offline)          compare-before-write, opening
+//                                                        the drain's undo span if no
+//                                                        switch already did (UndoSpan);
 //                                                        true iff it actually wrote
 //   void   releaseFx(guid)                               release the record's FX half and
 //                                                        arm the save-time flush that
@@ -213,8 +221,10 @@ ReclaimTickResult drainTick(std::vector<ReclaimItem>& items, ParkClock& clock, H
     auto nextAllowed = [&](double nowMs) {
         const int i = nextEligible(items, clock, nowMs);
         if (i >= 0 && !offlineAllowed &&
-            items[static_cast<std::size_t>(i)].work.kind == view::FxWorkKind::Offline)
+            items[static_cast<std::size_t>(i)].work.kind == view::FxWorkKind::Offline) {
+            result.transportDeferred = true;
             return -1;
+        }
         return i;
     };
 
@@ -274,6 +284,9 @@ struct ReclaimRunResult {
     int  writes  = 0;
     bool settled = false;
 
+    // Carried straight out of the tick — see ReclaimTickResult::transportDeferred.
+    bool transportDeferred = false;
+
     // Something invalidated the queue while the drain was on the stack. The work and
     // the releases are DISCARDED, `settled` is never true, and the caller must not go
     // idle — the next tick re-derives against whatever changed.
@@ -376,8 +389,18 @@ public:
         // back whether or not the drain's own work survives.
         clock_.swap(clock);
 
-        if (generation_ != entered)
-            return ReclaimRunResult{tick.writes, false, true, tick.drops};
+        // Assembled field by field rather than positionally: the aggregate is read by
+        // the undo span's close decision now, and a silently mis-slotted bool there
+        // leaks an open block.
+        ReclaimRunResult run;
+        run.writes            = tick.writes;
+        run.transportDeferred = tick.transportDeferred;
+        run.drops             = tick.drops;
+
+        if (generation_ != entered) {
+            run.aborted = true;
+            return run;
+        }
 
         items_.swap(work);
         // A release dropped by an abort is re-owed, not lost: the next derivation plans
@@ -385,7 +408,8 @@ public:
         // release fires then. Applied in MEMORY only — the spend reaches disk on the
         // project's next save (ReleaseFlush), never on the drain's own initiative.
         for (const std::string& guid : tick.released) host.releaseFx(guid);
-        return ReclaimRunResult{tick.writes, tick.settled, false, tick.drops};
+        run.settled = tick.settled;
+        return run;
     }
 
 private:
@@ -433,6 +457,87 @@ private:
     ReaProject* project_ = nullptr;  // COMPARE-ONLY, never dereferenced
 };
 
+// --- the undo span ---------------------------------------------------------------
+
+// The label a drain that no mode switch opened a span for closes under. Names the
+// feature the user knows — Design View, and its CPU reclaim (the toggle's own tooltip
+// wording) — rather than the loop behind it, and covers both directions: one act,
+// converging the parked/returning chains to what the view state says.
+inline constexpr const char* kReclaimUndoLabel = "ReaSampler: reclaim Design View FX";
+
+// One open REAPER undo block, held across the timer callbacks the drain runs on.
+//
+// DAW-measured this phase, and the whole reason this type exists: block WIDTH is the
+// entire effect. From a timer, ten TrackFX_SetOffline calls mint ten undo points
+// unblocked, ten with a block PER WRITE, and ONE under a single block spanning all
+// ten — the ("", 0) discard form does not discard outside an action, it coalesces. A
+// block opened on one timer callback and closed on a LATER one holds.
+//
+// Pure: the Undo_BeginBlock2/EndBlock2 calls are fx_reclaim.cpp's, which owns the ONE
+// instance and routes every path through one open and one close. Both moves report
+// whether they actually moved, so a caller may say either unconditionally and still
+// make no second BeginBlock2 and no unbalanced EndBlock2.
+class UndoSpan {
+public:
+    bool               open() const { return open_; }
+    ReaProject*        project() const { return project_; }
+    const std::string& label() const { return label_; }
+    int                mask() const { return mask_; }
+
+    // True iff this call opened it. `label`/`mask` are adopted only then, so a drain
+    // writing under a switch's span cannot displace the switch's own label.
+    bool begin(ReaProject* proj, std::string label, int mask) {
+        if (open_) return false;
+        project_ = proj;
+        label_   = std::move(label);
+        mask_    = mask;
+        open_    = true;
+        return true;
+    }
+
+    // A mode switch decides its label only after its writes (applyMintsUndoPoint).
+    void relabel(std::string label, int mask) {
+        if (!open_) return;
+        label_ = std::move(label);
+        mask_  = mask;
+    }
+
+    // Add state the span did not originally promise to serialize — the FX bit, once
+    // this drain has actually written FX under a switch's flags-and-items mask.
+    void widen(int extraMask) {
+        if (open_) mask_ |= extraMask;
+    }
+
+    // True iff this call closed it; the caller owes Undo_EndBlock2 with the values it
+    // read BEFORE calling, since this clears them.
+    bool end() {
+        if (!open_) return false;
+        project_ = nullptr;
+        label_.clear();
+        mask_  = 0;
+        open_  = false;
+        return true;
+    }
+
+private:
+    ReaProject* project_ = nullptr;  // passed back to Undo_EndBlock2, never dereferenced here
+    std::string label_;
+    int         mask_ = 0;
+    bool        open_ = false;
+};
+
+// Must the tick that produced `run` end the span?
+//
+// The span may stay open only across a wait whose length it can bound. Waiting out the
+// 3s debounce qualifies and is the wait the whole design turns on — that dead time sits
+// between the switch and its first FX write. A TRANSPORT deferral does not: it lasts as
+// long as the user plays, and a block held open for a playback pass swallows every edit
+// made during it. Ending there costs a second undo point for the parking that follows,
+// which is the documented degradation, not a failure.
+inline bool spanEndsAfter(const ReclaimRunResult& run) {
+    return run.settled || run.aborted || run.transportDeferred;
+}
+
 // --- shell entry points --------------------------------------------------------
 
 // The ONE test the timer makes per tick, mirroring the realtime capture's single
@@ -460,6 +565,24 @@ bool fxReclaimDrainInProgress();
 // park record's FX half — see FxReclaimQueue::run.
 void fxReclaimTick(ReaSamplerSession& session, ReaProject* proj);
 
+// Opens the undo span a mode switch and its background FX parking share, so the two are
+// ONE Ctrl-Z. Called by applyMode in place of its own Undo_BeginBlock2; a REAL switch
+// then returns with the span still open and the reclaimer closes it once the parking has
+// drained. No-op if a span is already open, so callers END first (see applyMode) rather
+// than nesting.
+void fxReclaimBeginUndoSpan(ReaProject* proj);
+
+// Sets what the eventual close names the point. Separate from the begin because the
+// switch's own label is only decided after its writes (applyMintsUndoPoint).
+void fxReclaimLabelUndoSpan(const char* label, int mask);
+
+// The ONE close, idempotent: every terminating path routes through it — the drain
+// settling, aborting, or deferring on the transport; the project retargeting or closing;
+// the next apply superseding it; a project save; extension unload. A leaked open block
+// would swallow everything the user does afterwards, so nothing may return past a
+// terminating condition without calling this.
+void fxReclaimEndUndoSpan();
+
 // Writes an in-memory release into the SAVING project's view state (identified via
 // GetCurrentProjectInLoadSave(), not the active tab). Called from the project-save
 // hook and nowhere else: this is the ONE path that makes a spent record durable, and

src/shell/view/view.cpp

@@ -2,7 +2,8 @@
 // reaper_plugin_functions.h without REAPERAPI_IMPLEMENT (main.cpp owns that).
 // Tree arithmetic lives in view_tree (pure), one track's park/restore in view_park
 // and the lane writes in view_lanes; this file owns the plan iteration, parent
-// visibility, and the one undo block they all ride.
+// visibility, and opening the one undo span they all ride (fx_reclaim's UndoSpan,
+// which a real switch hands to the reclaimer still open).
 
 #include "shell/view/view.h"
 
@@ -23,8 +24,6 @@
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_EnumProjects
 #define REAPERAPI_WANT_GetPlayStateEx
-#define REAPERAPI_WANT_Undo_BeginBlock2
-#define REAPERAPI_WANT_Undo_EndBlock2
 #define REAPERAPI_WANT_TrackList_AdjustWindows
 #define REAPERAPI_WANT_UpdateArrange
 #define REAPERAPI_WANT_UpdateTimeline
@@ -117,9 +116,13 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
 
     bool laneModeChanged = false;
     bool wrote = false;  // anything at all this apply changed — see applyMintsUndoPoint
-    std::vector<std::string> exposedNames;  // named while handles are fresh, reported after the block
+    std::vector<std::string> exposedNames;  // named while handles are fresh, reported after the writes
 
-    Undo_BeginBlock2(project);
+    // A prior switch's span, if the reclaimer has not closed it yet: this apply
+    // supersedes it, so that parking lands as its own point rather than nesting under
+    // ours. Both calls are idempotent — the end reaches nothing when none is open.
+    fxReclaimEndUndoSpan();
+    fxReclaimBeginUndoSpan(project);
     {
         UiRefreshHold uiHold;  // every write below lands with the TCP/MCP frozen
 
@@ -195,14 +198,20 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
     if (laneModeChanged) UpdateTimeline();
 
     const bool mint = applyMintsUndoPoint(realSwitch, wrote);
-    Undo_EndBlock2(project, mint ? undoLabel.c_str() : "", mint ? kViewUndoMask : 0);
+    fxReclaimLabelUndoSpan(mint ? undoLabel.c_str() : "", mint ? kViewUndoMask : 0);
 
-    // After the undo block: an EXPOSE report is a console message, not a project
-    // write, and must not join what a Ctrl-Z rolls back.
+    // A REAL switch returns with the span STILL OPEN: the reclaimer closes it once the
+    // parking this apply is about to invalidate has drained, so the switch and its FX
+    // writes are ONE Ctrl-Z. Everything else closes here as it always did — a reapply is
+    // not a switch, and the load tick must not leave a block open across a project open.
+    if (!realSwitch) fxReclaimEndUndoSpan();
+
+    // Neither of the two below is a project write — a console message and an in-memory
+    // fingerprint — so whether the span is still open over them is immaterial.
     reportExposedTracks(project, exposedNames);
 
     // The tool's own account of what it just left driven — the only one that survives
-    // an undo rolling the model back underneath it.
+    // an undo rolling the model back underneath it. In memory, never on disk.
     noteAppliedState(project, model);
 
     return true;

src/shell/view/view.h

@@ -32,9 +32,6 @@ bool transportBlocksModeSwitch(ReaProject* proj);
 //
 // A real switch mints regardless, fail-safe: it is an explicitly fired action and has
 // to stay undoable even in the degenerate case where the plan found nothing to write.
-// Note what applyMode CANNOT see when it decides — the `view_state` ext-state write
-// lands in persistViewState, after this block has closed (src/shell/view/CLAUDE.md's
-// Gotchas).
 inline bool applyMintsUndoPoint(bool realSwitch, bool wroteAnything) {
     return realSwitch || wroteAnything;
 }
@@ -54,6 +51,10 @@ bool modeApplyInProgress();
 // path: TrackFX_SetOffline was 1897ms of a measured 1942ms switch, against 0.02ms for
 // all the flag work below. Do not reintroduce a synchronous FX write.
 //
+// A REAL switch therefore RETURNS WITH ITS UNDO BLOCK STILL OPEN (fx_reclaim's
+// UndoSpan): the reclaimer closes it once the parking has drained, which is what keeps
+// the switch and its FX writes ONE point rather than two. A reapply closes its own.
+//
 // Returns false (no mutation) if `targetModeId` isn't registered, if this is a real
 // switch (target != active) while the transport is running, if it is re-entered
 // (modeApplyInProgress), or if the project does not validate. A reapply (target ==