Design View: no undo block may open over the reclaim span, and a reapply closes none

Every opener ends the span first, so a tag fired mid-drain no longer hijacks the
switch's point. One stated criterion governs both lane sites; no-op actions discard.

src/shell/actions/design_view_actions.cpp

@@ -25,6 +25,7 @@
 #include "shell/capture/item_read.h"     // shared MediaItem* -> GUID + fixed-lane-name reads (D2 W3-B)
 #include "shell/capture/track_guid.h"    // shared MediaTrack* -> canonical GUID key
 #include "shell/panel/panel_window.h"    // bankPanelInvalidate — footer toggle repaint
+#include "shell/view/fx_reclaim.h"       // fxReclaimEndUndoSpan — no block opens over the span
 #include "shell/view/view.h"             // applyMode (D2 shell)
 #include "shell/view/view_lanes.h"       // mintManagedLanes
 #include "shell/view/view_park.h"        // kViewUndoMask
@@ -105,12 +106,16 @@ std::vector<std::string> selectedTrackGuids() {
 }
 
 // Reapplies the CURRENT active mode so a membership mutation takes visible effect
-// immediately (park/unpark/re-derive parents). The return is deliberately discarded: a
+// immediately (park/unpark/re-derive parents), answering whether it wrote any project
-// refused reapply leaves the membership edit standing and unapplied, and the user's next
+// state — the caller's block mints only if something did. `applied` is deliberately
-// edit or mode switch reapplies it. What must NOT be dropped with it is the reclaimer's
+// discarded: a refused reapply leaves the membership edit standing and unapplied, and
-// queue, which is why applyMode invalidates on its refusal path too (view.cpp).
+// the user's next edit or mode switch reapplies it (a refusal writes nothing, so it
-void reapplyActiveMode() {
+// reports false here and the block discards). What must NOT be dropped with it is the
-    applyMode(g_session->view(), g_session->view().activeModeId(), nullptr);
+// reclaimer's queue, which is why applyMode invalidates on its refusal path too
+// (view.cpp).
+bool reapplyActiveMode() {
+    return applyMode(g_session->view(), g_session->view().activeModeId(), nullptr)
+        .wroteProjectState;
 }
 
 constexpr int kFreeModeFixedLanes = 2;  // I_FREEMODE value; mirrors the shell's constant
@@ -189,15 +194,21 @@ void doToggleMode() {
     const std::string target =
         nextModeId(g_session->view().modes(), g_session->view().activeModeId());
     if (target.empty()) return;  // no modes to cycle to (degenerate)
-    if (!applyMode(g_session->view(), target, nullptr)) { reportModeSwitchRefused(); return; }
+    if (!applyMode(g_session->view(), target, nullptr).applied) {
+        reportModeSwitchRefused();
+        return;
+    }
     persistViewState(PersistScope::ViewOnly);
     bankPanelInvalidate();  // repaint the footer [Arrange|Design] toggle immediately
 }
 
-// Direct jump to a named mode. applyMode is a no-op (returns false, no mutation) if
+// Direct jump to a named mode. applyMode is a no-op (`applied` false, no mutation) if
 // the id is unregistered or the transport is running, so both fail safe.
 void doActivateMode(const std::string& modeId) {
-    if (!applyMode(g_session->view(), modeId, nullptr)) { reportModeSwitchRefused(); return; }
+    if (!applyMode(g_session->view(), modeId, nullptr).applied) {
+        reportModeSwitchRefused();
+        return;
+    }
     persistViewState(PersistScope::ViewOnly);
     bankPanelInvalidate();  // repaint the footer [Arrange|Design] toggle immediately
 }
@@ -208,18 +219,34 @@ void doActivateMode(const std::string& modeId) {
 // actually fired rather than the reapply it happened to trigger. Every one of them
 // closes the block before persistViewState, which can pop a modal Save-As. Stack RAII
 // so a throw between the two calls cannot leave the block open in the user's project.
+//
+// ENDING THE RECLAIM SPAN IS PART OF OPENING, and every action here inherits it from
+// this one constructor. A mode switch fired seconds ago has left the span open; a block
+// opened over it makes each close pair with the other's open, collapsing the switch
+// into this tag — mechanism and full rule in src/shell/view/CLAUDE.md.
+//
+// `markWrote` decides whether this block mints at all: re-tagging tracks that already
+// sit in that mode writes nothing to the project, and the ("", 0) discard form is real
+// from a hand-fired action (DAW-measured, same doc). The membership move itself is
+// never in the point either way — persistViewState runs after the close.
 class ActionUndoBlock {
 public:
     ActionUndoBlock(std::string label, int mask) : label_(std::move(label)), mask_(mask) {
+        fxReclaimEndUndoSpan();
         Undo_BeginBlock2(nullptr);
     }
-    ~ActionUndoBlock() { Undo_EndBlock2(nullptr, label_.c_str(), mask_); }
+    ~ActionUndoBlock() {
+        if (wrote_) Undo_EndBlock2(nullptr, label_.c_str(), mask_);
+        else        Undo_EndBlock2(nullptr, "", 0);
+    }
+    void markWrote() { wrote_ = true; }
     ActionUndoBlock(const ActionUndoBlock&) = delete;
     ActionUndoBlock& operator=(const ActionUndoBlock&) = delete;
 
 private:
     std::string label_;
     int         mask_;
+    bool        wrote_ = false;
 };
 
 // tag() replaces any prior single-mode membership (a leaf lives in one mode; the
@@ -231,7 +258,7 @@ void doTag(const std::string& modeId) {
     {
         ActionUndoBlock undo("ReaSampler: tag selected tracks -> " + modeId, kViewUndoMask);
         for (const std::string& g : guids) g_session->view().membership().tag(g, modeId);
-        reapplyActiveMode();
+        if (reapplyActiveMode()) undo.markWrote();
     }
     persistViewState(PersistScope::Full);
 }
@@ -245,7 +272,7 @@ void doUntag() {
     {
         ActionUndoBlock undo("ReaSampler: untag selected tracks", kViewUndoMask);
         for (const std::string& g : guids) g_session->view().membership().untag(g);
-        reapplyActiveMode();
+        if (reapplyActiveMode()) undo.markWrote();
     }
     persistViewState(PersistScope::Full);
 }
@@ -260,7 +287,7 @@ void doShowBoth() {
         ActionUndoBlock undo("ReaSampler: toggle show-both on selected tracks", kViewUndoMask);
         MembershipIndex& m = g_session->view().membership();
         for (const std::string& g : guids) m.setShowBoth(g, !m.isShowBoth(g));
-        reapplyActiveMode();
+        if (reapplyActiveMode()) undo.markWrote();
     }
     persistViewState(PersistScope::Full);
 }
@@ -290,8 +317,11 @@ void doMoveItems(const std::string& targetMode) {
         }
         // Re-drive the same minting/apply path auto-tag uses: mint/split lanes for any
         // track whose items now span modes, then reassert active-mode lane visibility.
-        mintManagedLanes(g_session->view(), nullptr);
+        // Either half writing is enough to mint; a retag that split no lanes and drove
-        reapplyActiveMode();
+        // no flag leaves nothing for a point to hold.
+        const bool minted    = mintManagedLanes(g_session->view(), nullptr);
+        const bool reapplied = reapplyActiveMode();
+        if (minted || reapplied) undo.markWrote();
     }
 
     persistViewState(PersistScope::Full);

src/shell/capture/render_in_place.cpp

@@ -21,6 +21,7 @@
 #include "shell/capture/track_guid.h"            // guidString
 #include "shell/panel/panel_input.h"             // bankPanelTailSetting
 #include "shell/persist/session.h"
+#include "shell/view/fx_reclaim.h"               // fxReclaimEndUndoSpan
 #include "shell/view/view.h"                     // applyMode
 #include "shell/view/view_lanes.h"               // mintManagedLanes
 
@@ -133,6 +134,11 @@ void RunRenderTrackInPlace(ReaSamplerSession& session) {
     // visible — never to choose a tag. Both tags below are absolute.
     const std::string activeMode = session.view().activeModeId();
 
+    // A mode switch fired seconds ago has left the FX reclaimer's undo span open, and
+    // no block may open over it — mechanism in src/shell/view/CLAUDE.md. Ending it here
+    // mints the switch's point, whole, before this block exists; idempotent otherwise.
+    fxReclaimEndUndoSpan();
+
     Undo_BeginBlock2(nullptr);
 
     // flags = 0, never 1: flags&1 adds default envelopes/FX, and a default chain

src/shell/view/CLAUDE.md

@@ -74,11 +74,34 @@ decide membership or mode rules.
   block per write, and ONE under a single block spanning all ten — block WIDTH is the
   entire effect there. 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. So: a path that writes only flags opens NO
+  callback and closed on a LATER one holds.
-  block (a repair or a reapply must not cost the user a Ctrl-Z), and a path whose act
+  **The four flags above are the whole of what was measured.** The lane parameters this
-  IS the user's opens one to make that act undoable at all — which is why the three
+  directory also writes with no block behind them — `C_LANEPLAYS`, `I_FREEMODE`,
-  membership-edit actions own their own (`design_view_actions`' `ActionUndoBlock`)
+  `C_LANESCOLLAPSED`, `C_LANESETTINGS` (`view_lanes`' `applyLaneOps`) — are assumed
-  rather than leaning on a reapply that now mints nothing.
+  silent BY INFERENCE from the four, never observed: `[verify — DAW]`. If they turn out
+  to mint unaided, a reapply that repairs a lane leaves a stray point the criterion
+  below did not intend; that is the failure mode to look for.
+- **ONE CRITERION decides who opens an undo block: the path whose own act has to
+  survive as a point, and no other.** Two things make an act the path's own — it was
+  explicitly fired by the user (a real mode switch, the membership actions,
+  `render_in_place`), or it makes a STRUCTURAL project change no re-derivation
+  reproduces (`view_lanes`' `mintManagedLanes`, which creates lanes and re-homes items;
+  the panel's detect tick fires it, so it is nobody's action, and it opens a block
+  anyway because nothing else would ever put those lanes back). Everything else
+  RE-ASSERTS driven state the view model recomputes from membership on every apply —
+  the park/restore flags, parent visibility, and `applyLaneOps`' `C_LANEPLAYS` plus its
+  `I_FREEMODE` re-assert on a track that already owns managed lanes — and opens none,
+  because a block there could only manufacture the point a project open or a membership
+  edit must not cost. This is why the same `I_FREEMODE` write is blocked at one site and
+  not the other: minting SPLITS a track, `applyLaneOps` only re-states which lane plays.
+  Second clause, binding every site that does open one: **a block that wrote nothing
+  closes on the discard form** (`Undo_EndBlock2(proj, "", 0)` — real from a hand-fired
+  action, per the measurement above), so re-tagging tracks already in that mode leaves
+  no empty point. The one exception is a REAL SWITCH, which mints written state or not:
+  it is an explicitly fired action and has to stay undoable even where the plan found
+  nothing to write. "Wrote" means a REAPER write throughout — model moves persist
+  through `persistViewState`, outside every block, so no point could carry them
+  (`ModeApplyResult`, `view.h`).
 - **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
@@ -92,12 +115,37 @@ decide membership or mode rules.
   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; ANY PROJECT-STATE
+  project retargeting or closing; the next REAL SWITCH superseding it; ANY PROJECT-STATE
-  LOAD; a project save; extension unload. The span may stay open only across a wait it
+  LOAD; a project save; extension unload; and any path about to open a block of its own
+  (next invariant). 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`).
+- **NOTHING MAY OPEN AN UNDO BLOCK WHILE THE SPAN IS OPEN — every opener ends it
+  first, as its first act.** REAPER's blocks are read as a DEPTH COUNTER, not a stack
+  of identities (`[verify — DAW]`, and the same undocumented nested-block behaviour the
+  Gotchas below name): `Undo_EndBlock2` at depth 2->1 mints nothing and drops the label
+  and mask it was handed, and the close that does reach depth 0 mints under whatever
+  label IT carries. So a block opened over the span leaves each close paired with the
+  other's open — the switch's point gone, its parking relabelled as the intruder's act,
+  and the `UNDO_STATE_FX` the drain widened the span to (`UndoSpan::widen`) dropped,
+  which is the exact loss the widen exists to prevent. The RULE does not rest on that
+  reading: ending the span first yields the switch's point and the opener's as two
+  correctly-labelled points under the counter reading AND under an identity-pairing one,
+  which is why it is stated as an unconditional discipline rather than a workaround for
+  a behaviour nobody has observed. Ending it first mints the switch's point whole, at
+  depth 0, and leaves the opener's block the only one in play. The
+  openers, all of them: `design_view_actions`' `ActionUndoBlock` (whose constructor
+  ends the span, so all four membership actions inherit it), `view_lanes`'
+  `mintManagedLanes`, and `shell/capture`'s `render_in_place`. The mirror half is
+  `applyMode`'s: a REAPPLY never closes a span, precisely because it is reachable from
+  inside one of those blocks — only a real switch supersedes, and a real switch is
+  always fired at depth 0. RESIDUAL, out of this rule's reach: an extension undo block
+  ELSEWHERE (a capture, a bank verb, an ingest) that pumps the message loop can re-enter
+  the timer, and a drain tick that settles in there closes the span at depth 2->1 with
+  the same result. Closing that needs every extension-owned `Undo_BeginBlock2` to route
+  through one opener that ends the span — not attempted here.
 - **EVERY project-state load terminates the span itself, at the hook, before REAPER
   restores anything — `isUndo` does not narrow it.**
   `fxReclaimAbandonForStateLoad` runs UNBRANCHED from `BeginLoadProjectState`
@@ -137,13 +185,13 @@ decide membership or mode rules.
   rather than resuming on the next tick. (That a revert leaves the view model stale at
   all is the identity poll's defect, not the reclaimer's — this invariant only keeps the
   reclaimer from acting on it.)
-- **A reapply opens no undo block at all, so it can leave no point** — whether it
+- **A reapply opens no undo block and CLOSES NO SPAN, so it can leave no point** —
-  wrote or not, and whoever drove it: a project open, a membership edit,
+  whether it wrote or not, and whoever drove it: a project open, a membership edit,
-  `render_in_place`'s inner reapply. Nothing a reapply writes mints unaided (see the
+  `render_in_place`'s inner reapply. The open half is the criterion above (a reapply
-  measurement above), so a block there could only manufacture the Ctrl-Z opening a
+  re-asserts derived state and is nobody's act); the close half is the nesting rule
-  project must not cost. A caller whose own act needs to stay undoable spans it
+  above (a reapply runs inside its driver's block, so a supersede-close from there
-  itself. A REAL switch still always mints, written state or not: it is an explicitly
+  would land at depth 2->1 and hijack it). It reports what it wrote instead, and the
-  fired action and has to stay undoable even when the plan found nothing to write.
+  driver holding the block decides — `ModeApplyResult::wroteProjectState`.
 - **A rapid A→B→A flip moves zero plugins.** The reclaimer only starts a park's
   OFFLINE work once a track has read continuously parked for `kParkHoldMs`
   (3000ms). A flip inside that window flips the flags twice but the debounce
@@ -186,7 +234,7 @@ 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 on a REAL SWITCH opens the ONE undo span all of that rides (`fx_reclaim` is what closes it; a reapply opens none). 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 on a REAL SWITCH ends any prior span and opens the ONE undo span all of that rides (`fx_reclaim` is what closes it; a reapply opens none and closes none). Reports what it wrote (`ModeApplyResult`) so a caller holding its own block can discard an empty 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_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). It opens no undo block: it runs in the wake of a Ctrl-Z, and its writes are flags only.
@@ -288,12 +336,17 @@ applies the resulting lane state to live tracks.
   refusal leaves the membership tags it already wrote in place, so the next
   reapply — not a user re-fire of render-in-place — is what brings the tracks
   into sync.
-- `render_in_place` calls `applyMode` inside its own `Undo_BeginBlock2` pair, and
+- **"Nothing nests on the reapply paths" was FALSE, and cost the switch its undo
-  that block is now the only one in play on that path — the reapply opens none of
+  point.** `render_in_place` and the membership actions each call `applyMode` inside
-  its own, so nothing nests and the SDK's undocumented nested-block coalescing
+  their own `Undo_BeginBlock2` pair; the reapply opens no block of its own, but a prior
-  stopped being load-bearing here. The same holds for the membership actions'
+  REAL SWITCH's span can still be open — for `kParkHoldMs` plus the whole drain — and
-  `ActionUndoBlock`; `doMoveItems` is the one that still nests, over
+  it is a block like any other. Two things nest there, and both are now handled by the
-  `mintManagedLanes`.
+  "nothing may open a block while the span is open" invariant: the caller's block over
+  the span (each caller ends the span first), and the reapply's former unconditional
+  supersede-close inside the caller's block (only a real switch closes now). What still
+  nests legitimately: `doMoveItems` and `render_in_place` over `mintManagedLanes`, both
+  properly paired inside a single outer block, so the SDK's undocumented nested-block
+  coalescing is load-bearing only for that shape.
 - `fx_offline`'s identity keying (`TrackFX_GetFXGUID`) assumes the GUID stays
   attached to its plugin across a chain mutation while parked. That is
   `[verify — DAW]` (see `fxGuidString` in `view_park.cpp`) and SWS issue #802 is a

src/shell/view/view.cpp

@@ -63,19 +63,20 @@ bool modeApplyInProgress() { return g_applying; }
 
 namespace {
 
-bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaProject* proj) {
+ModeApplyResult applyModeImpl(ViewModeModel& model, const std::string& targetModeId,
-    if (g_applying) return false;  // see ApplyLatch; same fail-closed shape as the guards below
+                              ReaProject* proj) {
+    if (g_applying) return {};  // see ApplyLatch; same fail-closed shape as the guards below
     ApplyLatch latch;
 
     if (!model.modes().contains(targetModeId)) {
-        return false;  // reject before touching the project — no partial apply
+        return {};  // reject before touching the project — no partial apply
     }
 
     // Pinned ONCE: resolving "current project" independently at the begin and the end
     // would let a tab switch mid-apply open the block on one project and close it on
     // another.
     ReaProject* const project = proj ? proj : EnumProjects(-1, nullptr, 0);
-    if (!ValidatePtr2(nullptr, project, "ReaProject*")) return false;
+    if (!ValidatePtr2(nullptr, project, "ReaProject*")) return {};
 
     // The discriminator behind both halves of the contract in view.h. A gate placed
     // unconditionally here would break tagging and the project-load reapply during
@@ -85,7 +86,7 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
     const bool realSwitch = targetModeId != outgoingModeId;
 
     if (realSwitch && transportBlocksModeSwitch(project)) {
-        return false;  // same fail-closed shape as the mode-exists guard above
+        return {};  // same fail-closed shape as the mode-exists guard above
     }
 
     TrackHandles handleByGuid;
@@ -115,17 +116,20 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
         (targetMode ? targetMode->displayName : targetModeId) + " view";
 
     bool laneModeChanged = false;
+    bool wrote = false;  // REAPER writes only — see ModeApplyResult
     std::vector<std::string> exposedNames;  // named while handles are fresh, reported after the writes
 
-    // A prior switch's span, if the reclaimer has not closed it yet: this apply
+    // ONLY A REAL SWITCH, both halves — end a prior switch's still-open span, then open
-    // supersedes it, so that parking lands as its own point rather than nesting under
+    // this one's. A REAPPLY must do neither: it opens none because nothing it writes
-    // ours. Idempotent — it reaches nothing when none is open.
+    // mints unaided, and it CLOSES none because it is reachable from inside a caller's
-    fxReclaimEndUndoSpan();
+    // own block (a membership action, render_in_place), where a close would land at
-
+    // depth 2->1 and hijack the switch's point instead of minting it. A caller that
-    // ONLY a real switch opens one. Nothing a reapply writes mints on its own, so a
+    // opens a block ends the span itself, first, at depth 0 — see this directory's
-    // block around it could only manufacture the undo point a project open must not
+    // CLAUDE.md. Both calls are idempotent; do not reintroduce either unconditionally.
-    // cost — do not reintroduce one here (see this directory's CLAUDE.md).
+    if (realSwitch) {
-    if (realSwitch) fxReclaimBeginUndoSpan(project);
+        fxReclaimEndUndoSpan();
+        fxReclaimBeginUndoSpan(project);
+    }
     {
         UiRefreshHold uiHold;  // every write below lands with the TCP/MCP frozen
 
@@ -144,6 +148,7 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
             MediaTrack* tr = resolve(trackByGuid, guid);
             if (!tr) continue;  // stale/deleted GUID — prune
             const TrackParkResult r = parkTrack(model, guid, tr, tp.flags);
+            if (r.wrote) wrote = true;
             // Named here, not after the block: TrackList_AdjustWindows below can
             // invalidate the handle.
             if (r.exposed) exposedNames.push_back(trackDisplayName(tr));
@@ -154,13 +159,14 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
             const std::string& guid = tp.flags.front().guid;
             MediaTrack* tr = resolve(trackByGuid, guid);
             if (!tr) continue;  // stale/deleted GUID — prune
-            restoreTrack(model, guid, tr, tp.flags);
+            if (restoreTrack(model, guid, tr, tp.flags)) wrote = true;
         }
 
         // MANAGED LANES: the active mode's lane plays+shows, every other managed lane
         // is silenced+hidden. Empty on a D1-only project — byte-identical there.
         const LaneApplyResult lanes = applyLaneOps(trackByGuid, plan.lanes);
         laneModeChanged = lanes.freeMode;
+        if (lanes.wrote) wrote = true;
 
         // PARENT VISIBILITY (never parked): recomputed every toggle, never recorded.
         // Only the two visibility flags — never mainSend/FX on a parent.
@@ -170,8 +176,8 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
             MediaTrack* tr = resolve(trackByGuid, node.guid);
             if (!tr) continue;  // stale GUID — prune
             const double show = visible.count(node.guid) ? 1.0 : 0.0;
-            writeIfChanged(tr, "B_SHOWINTCP", show);
+            if (writeIfChanged(tr, "B_SHOWINTCP", show)) wrote = true;
-            writeIfChanged(tr, "B_SHOWINMIXER", show);
+            if (writeIfChanged(tr, "B_SHOWINMIXER", show)) wrote = true;
         }
 
         model.setActiveMode(targetModeId);
@@ -212,13 +218,14 @@ bool applyModeImpl(ViewModeModel& model, const std::string& targetModeId, ReaPro
     // an undo rolling the model back underneath it. In memory, never on disk.
     noteAppliedState(project, model);
 
-    return true;
+    return ModeApplyResult{true, wrote};
 }
 
 } // namespace
 
-bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject* proj) {
+ModeApplyResult applyMode(ViewModeModel& model, const std::string& targetModeId,
-    const bool applied = applyModeImpl(model, targetModeId, proj);
+                          ReaProject* proj) {
+    const ModeApplyResult result = applyModeImpl(model, targetModeId, proj);
 
     // Every membership, snapshot and active-mode mutation in the system reaches the
     // project through this call, so this is the whole invalidation surface: the
@@ -231,7 +238,7 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
     // else would drop a queue that now converges toward the mode the user just left.
     // The cost when the refusal followed no mutation is one re-derivation on the timer.
     invalidateFxReclaim();
-    return applied;
+    return result;
 }
 
 } // namespace reasampler

src/shell/view/view.h

@@ -27,6 +27,21 @@ bool transportBlocksModeSwitch(ReaProject* proj);
 // one-shot signal on a call that would be refused — see main.cpp's load glue.
 bool modeApplyInProgress();
 
+// What one applyMode did.
+//
+// `wroteProjectState` counts REAPER writes ONLY — a driven flag, a lane parameter,
+// a parent's visibility. Model moves (a snapshot stored, a record's flag half spent,
+// the active mode set) are deliberately excluded: they reach disk through
+// persistViewState, which every caller runs OUTSIDE its undo block, so no point can
+// carry them and counting them would only mint an empty one. It is the answer a
+// caller holding its own block needs for the discard decision (design_view_actions'
+// ActionUndoBlock); the solo pair cannot affect it, since only a real switch drives
+// solo and a real switch mints regardless.
+struct ModeApplyResult {
+    bool applied           = false;
+    bool wroteProjectState = false;
+};
+
 // Captures each about-to-park track's prior flag values into `model`, caches/clears
 // the outgoing mode's solo state and replays the incoming mode's, runs planToggle,
 // applies the park/restore flag writes plus managed-lane and parent-visibility
@@ -41,16 +56,20 @@ bool modeApplyInProgress();
 // 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 opens none, so a
-// caller that wants its own act undoable opens its own (design_view_actions does).
+// caller that wants its own act undoable opens its own (design_view_actions does) —
+// and, because REAPER's blocks are a depth counter, ENDS THE SPAN BEFORE IT OPENS
+// THAT BLOCK. A reapply never closes a span, precisely because it can be running
+// inside such a caller's block.
 //
-// Returns false (no mutation) if `targetModeId` isn't registered, if this is a real
+// `applied` is false (no mutation) if `targetModeId` isn't registered, if this is a
-// switch (target != active) while the transport is running, if it is re-entered
+// 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 ==
 // active) is never gated and never touches solo. `proj` == nullptr means the current
 // project, resolved ONCE and used for every call this apply makes.
 //
 // Either way — applied or refused — this drops the FX reclaimer's derived queue, which
 // is why callers may safely discard the return after mutating the model.
-bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject* proj);
+ModeApplyResult applyMode(ViewModeModel& model, const std::string& targetModeId,
+                          ReaProject* proj);
 
 } // namespace reasampler

src/shell/view/view_lanes.cpp

@@ -16,6 +16,7 @@
 #include "core/view/lane_keys.h"
 #include "core/view/view_tree.h"
 #include "shell/capture/item_read.h"
+#include "shell/view/fx_reclaim.h"  // fxReclaimEndUndoSpan — no block opens over the span
 #include "shell/view/view_park.h"  // writeIfChanged — the read-before-write every driven parm takes
 
 #define REAPERAPI_MINIMAL
@@ -276,6 +277,11 @@ bool mintManagedLanes(ViewModeModel& model, ReaProject* proj) {
 
     const TrackByGuid trackByGuid = indexHandles(handleByGuid);
 
+    // No block may open over a still-open reclaim span — mechanism in this directory's
+    // CLAUDE.md. Placed past the empty-plan return above deliberately: a detect tick
+    // that mints nothing must not shorten a span it never competes with.
+    fxReclaimEndUndoSpan();
+
     // `proj` deliberately NOT pinned the way applyMode pins it: nothing between this
     // begin and its end pumps the message loop, so no tab switch can land between them.
     Undo_BeginBlock2(proj);

src/shell/view/view_lanes.h

@@ -15,8 +15,8 @@ class ReaProject;
 namespace reasampler {
 
 // `freeMode` alone decides whether UpdateTimeline is owed (SDK: required after an
-// I_FREEMODE change); `wrote` is any project write at all, which is what the apply's
+// I_FREEMODE change); `wrote` is a REAPER write, the reading ModeApplyResult
-// undo-point decision reads.
+// (view.h) carries through to the caller's undo-point decision.
 struct LaneApplyResult {
     bool freeMode = false;
     bool wrote    = false;
@@ -24,14 +24,22 @@ struct LaneApplyResult {
 
 // Reconciles each op's durable laneKey to the track's current ordinal (a
 // stale/renamed/deleted key is pruned, never mis-driven) and drives C_LANEPLAYS.
+//
+// Opens NO undo block, where mintManagedLanes below opens one over the same
+// parameters — the one criterion this directory's CLAUDE.md states, applied: this
+// re-asserts derived state the view model recomputes from membership on every apply,
+// so nothing here needs to survive as a point of its own; minting creates lanes and
+// re-homes items, which no re-derivation reproduces.
 LaneApplyResult applyLaneOps(const TrackByGuid& handleByGuid,
                              const std::vector<LanePlayOp>& lanes);
 
 // Splits any track visible in more than one mode while carrying its own media into
 // fixed lanes (one managed lane per involved mode), assigns items, and records
-// ownership in `model`. Wraps the mutation in one Undo block when non-empty. Returns
+// ownership in `model`. Wraps the mutation in one Undo block when non-empty — see
-// true if any lane was minted (repaint hint). `proj` == nullptr means the current
+// applyLaneOps above for why this site opens one and that one does not — and ENDS THE
-// project.
+// RECLAIM SPAN first, since REAPER's blocks are a depth counter and this is reachable
+// from a timer tick with a prior switch's span still open. Returns true if any lane
+// was minted (repaint hint). `proj` == nullptr means the current project.
 bool mintManagedLanes(ViewModeModel& model, ReaProject* proj);
 
 // Recovers managed-lane ownership from durable P_LANENAME on project open —

src/shell/view/view_park.cpp

@@ -144,7 +144,6 @@ TrackParkResult parkTrack(ViewModeModel& model, const std::string& guid, MediaTr
             snap.fxKeying  = prior->fxKeying;
         }
         model.storeSnapshot(guid, snap);
-        result.wrote = true;
     }
 
     if (applyFlags(tr, flags)) result.wrote = true;
@@ -153,9 +152,9 @@ TrackParkResult parkTrack(ViewModeModel& model, const std::string& guid, MediaTr
 
 bool restoreTrack(ViewModeModel& model, const std::string& guid, MediaTrack* tr,
                   const std::vector<TrackFlagOp>& flags) {
-    applyFlags(tr, flags);
+    const bool wrote = applyFlags(tr, flags);
     model.releaseSnapshotFlags(guid);
-    return true;
+    return wrote;
 }
 
 std::string trackDisplayName(MediaTrack* tr) {

src/shell/view/view_park.h

@@ -93,11 +93,16 @@ extern const int kViewUndoMask;
 // Returns whether it wrote.
 bool writeIfChanged(MediaTrack* tr, const char* parm, double value);
 
-// What one parkTrack call did. `wrote` is any project write at all — a flag write
+// What one parkTrack call did. `wrote` is a REAPER write — a driven flag moved —
-// or a record write — and feeds the apply's undo-point fold. `exposed` is true iff
+// and nothing else: storing the snapshot is a MODEL move, and the model reaches disk
-// this call handed the track back visible instead of parking or adopting it; the
+// outside every undo block, so counting it would mint a point carrying none of it
-// caller (view.cpp) collects exposed tracks across the whole apply for the one
+// (ModeApplyResult, view.h). The two readings part company on exactly one path — a
-// batched console report.
+// RE-PARK whose restore had already handed the flags back at park-shaped values, which
+// stores a fresh snapshot and writes no flag — and the REAPER reading is the one that
+// wants: nothing in the project moved, so nothing is there for a point to hold.
+// `exposed` is true iff this call handed the track back visible instead of parking or
+// adopting it; the caller (view.cpp) collects exposed tracks across the whole apply for
+// the one batched console report.
 struct TrackParkResult {
     bool wrote   = false;
     bool exposed = false;
@@ -170,8 +175,9 @@ inline bool shouldReport(bool sameOwnerAsLast, const std::vector<std::string>& l
 void reportExposedTracks(ReaProject* proj, const std::vector<std::string>& trackNames);
 
 // Restores one returning leaf from its record and spends the record's flag half.
-// Always true: the record is consumed here, so this apply changed state and is not
+// Returns whether a FLAG actually moved — the same REAPER-writes-only reading
-// repeatable, which is exactly what its one reader (the undo-point decision) asks.
+// TrackParkResult::wrote carries, and for the same reason: spending the record is a
+// model move no undo point can hold.
 bool restoreTrack(ViewModeModel& model, const std::string& guid, MediaTrack* tr,
                   const std::vector<TrackFlagOp>& flags);