Defer Design View's per-FX park to an idle tick; honest undo mask, compare-before-write, PreventUIRefresh bracket, O(1) handle resolve

src/app/CMakeLists.txt

@@ -38,6 +38,7 @@ add_library(reaper_reasampler MODULE
     ${LICE_SRC}
     ${REASAMPLER_SRC_DIR}/shell/capture/insert.cpp
     ${REASAMPLER_SRC_DIR}/shell/view/view.cpp
+    ${REASAMPLER_SRC_DIR}/shell/view/view_fx_park.cpp
     ${REASAMPLER_SRC_DIR}/shell/view/view_solo.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/track_guid.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/provenance_shell.cpp
@@ -59,6 +60,12 @@ target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model
 # link graph free of the voice engine — a link edge to it here means the design drifted.
 target_include_directories(reaper_reasampler PRIVATE ${SDK_INC} ${WDL_INC})
 
+# The deferred FX-park queue's re-entrancy rule is pure (header-inline, no REAPER
+# types), so it is CTest-covered like a core/ module. Declared here rather than in a
+# src/shell/view/CMakeLists.txt because that directory deliberately has none — its
+# TUs are compiled into this target directly.
+reasampler_test(view_fx_park LINK fx_offline)
+
 # Bank-package import: the promptless verb plus its action skin. Kept as its own
 # appended block rather than merged into the lists above, so the two package
 # directions stay textually independent.

src/app/main.cpp

@@ -38,6 +38,7 @@
 #include "shell/panel/panel_window.h"            // panel lifecycle (init/toggle/open-query/shutdown)
 #include "shell/persist/session.h"               // ReaSamplerSession
 #include "shell/view/view.h"                     // reconcileManagedLanes / applyMode
+#include "shell/view/view_fx_park.h"             // the mode switch's deferred FX park
 
 namespace capture = reasampler::capture;
 
@@ -172,6 +173,12 @@ static void OnTimer()
 
     g_session.poll();
 
+    // A mode switch applies its visibility/routing writes synchronously and leaves
+    // the per-FX offline work here, so the new mode paints before the plugins
+    // unload. Drained AFTER poll() so a project switch discards the queue instead
+    // of applying it to the project that replaced it; idle cost is one empty test.
+    reasampler::drainDeferredFxParks(g_session.view());
+
     // persist stays MODEL-ONLY (loads the saved view model but does not apply
     // visibility, to avoid coupling persist to the view shell); poll() raises a
     // one-shot load signal that we drain here to reapply the SAVED active mode so a

src/shell/view/CLAUDE.md

@@ -45,9 +45,16 @@ decide membership or mode rules.
   mode ids — no absolute paths, no index positions).
 - **Documented caveat:** offlined FX re-instantiate when a track returns to the
   active mode — stateful plugins (convolution, loaded samplers, tail-holding
-  effects) re-initialize on return (possible load hitch, un-persisted internal
-  state lost). Accepted cost of the CPU reclaim; surfaced at the toggle affordance
-  (tooltip).
+  effects) re-initialize on return (load hitch, un-persisted internal state lost).
+  Accepted cost of the CPU reclaim; surfaced at the toggle affordance (tooltip).
+  **The hitch no longer sits on the switch's synchronous path:** per-FX
+  offline/online is enqueued and applied on a later idle tick (`view_fx_park`),
+  so the new mode paints first. The deferral changes only WHEN the plugins move —
+  they still unload and re-instantiate, and un-persisted internal state is still
+  lost. What it does change is the undo record: the offline writes land outside
+  the switch's undo block, so the tool no longer re-drives them on an undo or a
+  redo — what a Ctrl-Z then leaves the chain at is REAPER's own FX-state record,
+  `[verify — DAW]`.
 - **Show-both semantics:** a per-track "pin visible across modes" flag re-enables
   processing whenever shown. A show-both leaf appears in every mode's visible set
   and is never parked — its driven flags stay at snapshot/restored values, FX
@@ -85,7 +92,8 @@ applies the resulting lane state to live tracks.
 
 ## Modules
 
-- `view` — Design View shell: snapshots flag values before parking, drives hide + CPU-park on inactive-mode leaves (`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN` + per-FX offline), restores from snapshot. 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`.**
+- `view` — Design View shell: snapshots flag values before parking, drives hide + CPU-park on inactive-mode leaves (`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN`, with per-FX offline deferred to `view_fx_park`), restores from snapshot. 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`.**
+- `view_fx_park` — the per-FX offline surface: the `TrackFX_GetFXGUID` identity read snapshot/park/restore share, the deferred intent queue that keeps `TrackFX_SetOffline` off the switch's synchronous path (at most one intent per track GUID, latest wins, an intent landing on its own pending inverse cancels it), and the idle-tick drain `main.cpp`'s `OnTimer` calls. The drain is also where a consumed snapshot is cleared — not where the restore was planned — so a second switch arriving before the first drained re-parks against the still-true captured state instead of re-capturing parked values.
 - `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`.
 
 ## Gotchas
@@ -105,7 +113,7 @@ applies the resulting lane state to live tracks.
   the solo cache inherits it rather than introducing it. Not fixed here.
 - `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.cpp`) and SWS issue #802 is a
+  `[verify — DAW]` (see `fxGuidString` in `view_fx_park.cpp`) and SWS issue #802 is a
   known reason it might not hold: `SNM_MoveOrRemoveTrackFX` reportedly leaves
   the FXID lines behind on reorder rather than moving them with the plugin. If
   confirmed, an SWS-driven reorder of a parked track's chain — not a native

src/shell/view/view.cpp

@@ -10,6 +10,7 @@
 #include <optional>
 #include <set>
 #include <string>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -18,6 +19,7 @@
 #include "core/view/lane_keys.h"
 #include "core/view/solo_cache.h"
 #include "shell/capture/track_guid.h"
+#include "shell/view/view_fx_park.h"
 #include "shell/view/view_solo.h"
 #include "core/view/view_tree.h"
 
@@ -28,12 +30,8 @@
 #define REAPERAPI_WANT_GetMediaTrackInfo_Value
 #define REAPERAPI_WANT_SetMediaTrackInfo_Value
 #define REAPERAPI_WANT_GetSetMediaTrackInfo_String
-#define REAPERAPI_WANT_ShowConsoleMsg
-#define REAPERAPI_WANT_TrackFX_GetCount
-#define REAPERAPI_WANT_TrackFX_GetFXGUID
+#define REAPERAPI_WANT_PreventUIRefresh
 #define REAPERAPI_WANT_TrackFX_GetOffline
-#define REAPERAPI_WANT_TrackFX_SetOffline
-#define REAPERAPI_WANT_guidToString
 #define REAPERAPI_WANT_Undo_BeginBlock2
 #define REAPERAPI_WANT_Undo_EndBlock2
 #define REAPERAPI_WANT_TrackList_AdjustWindows
@@ -64,6 +62,26 @@ constexpr int kFreeModeFixedLanes = 2;
 // deliberately excluded, nothing is moving there.
 constexpr int kTransportMoving = 1 | 4;
 
+// The domains one apply actually writes: track config/routing (visibility,
+// B_MAINSEND, I_SOLO), item lane assignment, and the ext-state block
+// (UNDO_STATE_MISCCFG covers extension state). NOT UNDO_STATE_ALL, which
+// includes UNDO_STATE_FX and so makes every undo record carry every track's FX
+// state — a cost that scales with the project's plugin count rather than with
+// what the switch changed. FX is OR'd back in per apply, only when an FX-domain
+// flag really moved.
+constexpr int kApplyUndoMask = UNDO_STATE_TRACKCFG | UNDO_STATE_ITEMS | UNDO_STATE_MISCCFG;
+
+// Holds REAPER's UI refresh off for the write phase; the deliberate rebuild
+// (TrackList_AdjustWindows + UpdateArrange) runs after it releases. RAII because
+// an unbalanced pair leaves the user's UI frozen with no way back — the SDK's
+// own warning at reaper_plugin_functions.h:5581.
+struct UiRefreshHold {
+    UiRefreshHold() { PreventUIRefresh(1); }
+    ~UiRefreshHold() { PreventUIRefresh(-1); }
+    UiRefreshHold(const UiRefreshHold&) = delete;
+    UiRefreshHold& operator=(const UiRefreshHold&) = delete;
+};
+
 // C_LANESCOLLAPSED=2: render a tool-split track like a normal single-lane
 // track showing only the playing lane (SDK: 1=collapsed, 2=hidden-lanes-exist
 // but displays as non-fixed-lane).
@@ -102,7 +120,7 @@ const char* flagParm(Flag f) {
 // construction. Also caches each MediaTrack* by GUID for later resolve().
 std::vector<TrackFolderEntry> readFolderEntries(
         ReaProject* proj,
-        std::vector<std::pair<std::string, MediaTrack*>>& handleByGuid) {
+        TrackHandles& handleByGuid) {
     std::vector<TrackFolderEntry> entries;
     int count = CountTracks(proj);
     entries.reserve(static_cast<std::size_t>(count));
@@ -119,46 +137,21 @@ std::vector<TrackFolderEntry> readFolderEntries(
     return entries;
 }
 
-MediaTrack* resolve(const std::vector<std::pair<std::string, MediaTrack*>>& handleByGuid,
-                    const std::string& guid) {
-    for (const auto& kv : handleByGuid) {
-        if (kv.first == guid) return kv.second;
-    }
-    return nullptr;  // stale/deleted GUID — pruned by being skipped
+// Every park, restore, parent and lane op resolves its GUID; a linear scan made
+// that O(T²) on the one path whose cost the user waits on. The vector form
+// survives beside it because view_solo's writers take one pass over it.
+using TrackByGuid = std::unordered_map<std::string, MediaTrack*>;
+
+TrackByGuid indexHandles(const TrackHandles& handleByGuid) {
+    TrackByGuid byGuid;
+    byGuid.reserve(handleByGuid.size());
+    for (const auto& kv : handleByGuid) byGuid.emplace(kv.first, kv.second);
+    return byGuid;
 }
 
-// The FX's own durable identity, braced exactly like the track GUID keys. Empty
-// when REAPER reports none — an FX we cannot name is one we cannot restore, and
-// fx_offline treats it that way rather than guessing at its slot. Lifetime is
-// settled: the string copy is taken immediately and the GUID* is never held
-// past this call (reaper_plugin_functions.h:7348 documents no null contract for
-// TrackFX_GetFXGUID; treating null as "no identity" is the safe read).
-//
-// [verify — DAW] STABILITY across a chain mutation is not settled the same way:
-// confirm the GUID for one FX instance survives a native drag-reorder, an SWS
-// move (SNM_MoveOrRemoveTrackFX — SWS issue #802 reports the FXID lines do not
-// follow the plugin after that call, i.e. wrong-plugin restores or mass drops
-// through fx_offline on that path specifically), a save/reload round trip, and
-// two live instances of one plugin type staying distinguishable. See
-// src/shell/view/CLAUDE.md's Gotchas for the SWS-path risk this leaves open.
-std::string fxGuidString(MediaTrack* tr, int fx) {
-    GUID* g = TrackFX_GetFXGUID(tr, fx);
-    if (!g) return {};
-    char buf[64] = {0};  // guidToString needs a >=64-char destination (SDK contract)
-    guidToString(g, buf);
-    return std::string(buf);
-}
-
-// The chain as it stands now: identity by current slot. Snapshot, park and
-// restore all address FX through this one plain 0..TrackFX_GetCount-1
-// enumeration — never the 0x1000000/0x2000000 input-FX or container forms — so
-// whatever it covers, all three cover identically.
-std::vector<std::string> liveFxGuids(MediaTrack* tr) {
-    const int fxCount = TrackFX_GetCount(tr);
-    std::vector<std::string> guids;
-    guids.reserve(static_cast<std::size_t>(fxCount));
-    for (int fx = 0; fx < fxCount; ++fx) guids.push_back(fxGuidString(tr, fx));
-    return guids;
+MediaTrack* resolve(const TrackByGuid& byGuid, const std::string& guid) {
+    auto it = byGuid.find(guid);
+    return it == byGuid.end() ? nullptr : it->second;  // stale/deleted GUID — pruned by being skipped
 }
 
 // Captures prior driven-flag state before parking. Never reads B_MUTE/I_SOLO;
@@ -179,30 +172,25 @@ TrackSnapshot snapshotTrack(MediaTrack* tr) {
     return snap;  // fxKeying stays Identity — a live capture always knows the chain
 }
 
-void applyFlags(MediaTrack* tr, const std::vector<TrackFlagOp>& flags) {
+// A reapply (tag/untag, project load) re-plans every flag it already applied, so
+// most of what it writes is a value the track already holds; the read is the
+// cheap half of the pair, and it also keeps those no-op writes out of the undo
+// record's mask.
+bool writeIfChanged(MediaTrack* tr, const char* parm, double value) {
+    if (GetMediaTrackInfo_Value(tr, parm) == value) return false;
+    SetMediaTrackInfo_Value(tr, parm, value);
+    return true;
+}
+
+void applyFlags(MediaTrack* tr, const std::vector<TrackFlagOp>& flags, int& undoMask) {
     for (const TrackFlagOp& op : flags) {
-        SetMediaTrackInfo_Value(tr, flagParm(op.flag), static_cast<double>(op.value));
+        if (!writeIfChanged(tr, flagParm(op.flag), static_cast<double>(op.value))) continue;
+        // I_FXEN is the only FX-domain flag driven here, so it is the only one
+        // whose undo record has to carry UNDO_STATE_FX.
+        if (op.flag == Flag::FxEnable) undoMask |= UNDO_STATE_FX;
     }
 }
 
-// The pure park plan leaves fxOffline empty by design; expand it here from the
-// live FX count.
-void parkFxOffline(MediaTrack* tr) {
-    int fxCount = TrackFX_GetCount(tr);
-    for (int fx = 0; fx < fxCount; ++fx) {
-        TrackFX_SetOffline(tr, fx, true);
-    }
-}
-
-// Restores per-FX offline from the snapshot verbatim — never a blanket "online".
-// Which live FX each captured state belongs to is resolveFxRestore's call, and
-// what it could not place comes back for the caller to report.
-FxRestoreDrops restoreFxOffline(MediaTrack* tr, const std::vector<FxOfflineOp>& fxOffline) {
-    const FxRestoreResolution res = resolveFxRestore(fxOffline, liveFxGuids(tr));
-    for (const FxOfflineWrite& w : res.writes) TrackFX_SetOffline(tr, w.fxIndex, w.offline);
-    return res.drops;
-}
-
 // Managed-lane application: the pure planner keys LanePlayOps by the lane's
 // DURABLE name; REAPER's C_LANEPLAYS:N is keyed by current ordinal, which
 // renumbers on reorder. So every write here re-resolves durable key -> current
@@ -238,7 +226,7 @@ std::map<std::string, int> managedLaneOrdinals(MediaTrack* tr) {
 void applyLanePlays(MediaTrack* tr, int laneIdx, int lanePlays) {
     char parm[32];
     std::snprintf(parm, sizeof(parm), "C_LANEPLAYS:%d", laneIdx);
-    SetMediaTrackInfo_Value(tr, parm, static_cast<double>(lanePlays));
+    writeIfChanged(tr, parm, static_cast<double>(lanePlays));
 }
 
 // Groups ops by track, reconciles each op's durable laneKey to the track's
@@ -246,7 +234,7 @@ void applyLanePlays(MediaTrack* tr, int laneIdx, int lanePlays) {
 // enables fixed-lane mode on any track carrying a managed lane, and drives
 // C_LANEPLAYS. UpdateTimeline() is the caller's job when this returns true
 // (SDK: required after an I_FREEMODE change).
-bool applyLaneOps(const std::vector<std::pair<std::string, MediaTrack*>>& handleByGuid,
+bool applyLaneOps(const TrackByGuid& handleByGuid,
                   const std::vector<LanePlayOp>& lanes) {
     if (lanes.empty()) return false;
 
@@ -312,7 +300,7 @@ std::string itemModeFromMembership(const ViewModeModel& model, const std::string
 // there regardless of name).
 std::vector<LaneTrack> readLaneTracks(
         const ViewModeModel& model,
-        const std::vector<std::pair<std::string, MediaTrack*>>& handleByGuid) {
+        const TrackHandles& handleByGuid) {
     std::vector<LaneTrack> tracks;
     tracks.reserve(handleByGuid.size());
     for (const auto& [guid, tr] : handleByGuid) {
@@ -358,7 +346,7 @@ bool assignItemToLane(MediaTrack* tr, MediaItem* it, int laneOrdinal) {
 // managed-eligible items; I_NUMFIXEDLANES is only ever GROWN, never shrunk,
 // so a user's existing manual lanes are never renamed or reassigned.
 bool applyMintPlan(ViewModeModel& model, const LaneMintPlan& plan,
-                   const std::vector<std::pair<std::string, MediaTrack*>>& handleByGuid) {
+                   const TrackByGuid& handleByGuid) {
     bool changed = false;
 
     std::map<std::string, std::vector<const LaneMint*>> mintsByTrack;
@@ -436,8 +424,9 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
         return false;  // same fail-closed shape as the mode-exists guard above
     }
 
-    std::vector<std::pair<std::string, MediaTrack*>> handleByGuid;
+    TrackHandles handleByGuid;
     std::vector<TrackFolderEntry> entries = readFolderEntries(proj, handleByGuid);
+    const TrackByGuid trackByGuid = indexHandles(handleByGuid);
     FolderTree tree = buildFolderTree(entries);
 
     // Prune snapshots for tracks no longer in the live enumeration before
@@ -455,81 +444,6 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
 
     TogglePlan plan = model.planToggle(tree, targetModeId);
 
-    Undo_BeginBlock2(proj);
-
-    // DISJOIN THE SOLO SURFACES. Both this clear and the replay after setActiveMode
-    // ride the existing undo block — one mode toggle stays one Ctrl-Z.
-    if (realSwitch) {
-        model.soloCache().store(outgoingModeId, outgoingSolo);
-        clearTrackSolos(handleByGuid, outgoingSolo);
-    }
-
-    // PARK: snapshot before mutating, store into the model, then apply.
-    for (const TrackPlan& tp : plan.park) {
-        if (tp.flags.empty()) continue;  // every op in a TrackPlan targets one track
-        const std::string& guid = tp.flags.front().guid;
-        MediaTrack* tr = resolve(handleByGuid, guid);
-        if (!tr) continue;  // stale GUID — prune
-
-        // Snapshot ONCE, at first park: a snapshot already present means the
-        // track is still parked from a prior apply, so its live flags are the
-        // parked values — recapturing would overwrite the true pre-park state
-        // with zeros and a later restore would hide it for good. Restore
-        // clears the snapshot, so the next genuine park recaptures fresh state.
-        if (model.snapshot(guid) == nullptr)
-            model.storeSnapshot(guid, snapshotTrack(tr));
-        applyFlags(tr, tp.flags);
-        parkFxOffline(tr);
-    }
-
-    // RESTORE: apply verbatim, then drop the consumed snapshot.
-    FxRestoreDrops fxDrops;
-    int fxDropTracks = 0;
-    for (const TrackPlan& tp : plan.restore) {
-        if (tp.flags.empty()) continue;
-        const std::string& guid = tp.flags.front().guid;
-        MediaTrack* tr = resolve(handleByGuid, guid);
-        if (!tr) continue;  // stale GUID — prune
-
-        applyFlags(tr, tp.flags);
-        const FxRestoreDrops drops = restoreFxOffline(tr, tp.fxOffline);
-        if (drops.total() > 0) {
-            fxDrops.add(drops);
-            ++fxDropTracks;
-        }
-        model.clearSnapshot(guid);
-    }
-
-    // Captured FX state that could not be applied is REPORTED. Silence here would
-    // read to the user as "restore worked" while an FX sat at whatever state the
-    // park left it in. Sent with the "!SHOW:" prefix (reaper_plugin_functions.h:6536)
-    // so it never force-opens the console window: applyMode's reapply path also
-    // runs unattended on project load (see the reconcile comment above), and this
-    // one call site can't tell that case apart from an interactive toggle/tag-edit
-    // reapply — both call in with target == active — so splitting loud-on-toggle
-    // from quiet-on-load would need a flag threaded from every caller, several of
-    // which are outside this change. Quiet-always is the safe default: the message
-    // still lands in the console for whoever opens it, on every path.
-    const std::string fxDropMsg = describeFxRestoreDrops(fxDrops, fxDropTracks);
-    if (!fxDropMsg.empty()) ShowConsoleMsg(("!SHOW:" + fxDropMsg).c_str());
-
-    // MANAGED LANES: drive C_LANEPLAYS so the active mode's lane plays+shows
-    // and every other managed lane is silenced+hidden. Empty for a D1-only
-    // project, leaving that behavior byte-identical.
-    const bool laneModeChanged = applyLaneOps(handleByGuid, plan.lanes);
-
-    // PARENT VISIBILITY (never parked): recomputed every toggle, never
-    // snapshotted. Only the two visibility flags — never mainSend/FX on a parent.
-    std::set<std::string> visible = model.visibleTracks(tree, targetModeId);
-    for (const FolderNode& node : tree.nodes) {
-        if (!node.isParent) continue;
-        MediaTrack* tr = resolve(handleByGuid, node.guid);
-        if (!tr) continue;  // stale GUID — prune
-        double show = visible.count(node.guid) ? 1.0 : 0.0;
-        SetMediaTrackInfo_Value(tr, "B_SHOWINTCP", show);
-        SetMediaTrackInfo_Value(tr, "B_SHOWINMIXER", show);
-    }
-
     // Target is guaranteed registered (checked at entry); fall back to the id
     // defensively if that ever changes.
     const Mode* targetMode = model.modes().query(targetModeId);
@@ -537,16 +451,81 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
         "ReaSampler: activate " +
         (targetMode ? targetMode->displayName : targetModeId) + " view";
 
-    model.setActiveMode(targetModeId);
+    int undoMask = kApplyUndoMask;
+    bool laneModeChanged = false;
 
-    // Replay + consume, before the single relayout below picks the change up. Dropped
-    // against `visible` (computed above for parent visibility), not the park plan: a
-    // folder parent can be hidden without being parked, and a hidden track must not
-    // receive a replayed solo it carries no visible control to undo.
-    if (realSwitch) {
-        if (const std::map<std::string, int>* cached = model.soloCache().query(targetModeId)) {
-            restoreTrackSolos(handleByGuid, *cached, liveGuids, visible);
-            model.soloCache().clear(targetModeId);
+    Undo_BeginBlock2(proj);
+    {
+        UiRefreshHold uiHold;  // every write below lands with the TCP/MCP frozen
+
+        // DISJOIN THE SOLO SURFACES. Both this clear and the replay after
+        // setActiveMode ride the existing undo block — one mode toggle stays one Ctrl-Z.
+        if (realSwitch) {
+            model.soloCache().store(outgoingModeId, outgoingSolo);
+            clearTrackSolos(handleByGuid, outgoingSolo);
+        }
+
+        // PARK: snapshot before mutating, store into the model, then apply. The
+        // per-FX offline half is deferred (view_fx_park) — it is the expensive
+        // half and nothing about the new mode's appearance waits on it.
+        for (const TrackPlan& tp : plan.park) {
+            if (tp.flags.empty()) continue;  // every op in a TrackPlan targets one track
+            const std::string& guid = tp.flags.front().guid;
+            MediaTrack* tr = resolve(trackByGuid, guid);
+            if (!tr) continue;  // stale GUID — prune
+
+            // Snapshot ONCE, at first park: a snapshot already present means the
+            // track is still parked from a prior apply (or its deferred restore
+            // has not landed yet), so its live flags are the parked values —
+            // recapturing would overwrite the true pre-park state with zeros and
+            // a later restore would hide it for good.
+            if (model.snapshot(guid) == nullptr)
+                model.storeSnapshot(guid, snapshotTrack(tr));
+            applyFlags(tr, tp.flags, undoMask);
+            deferFxPark(proj, guid);
+        }
+
+        // RESTORE: flags verbatim now, per-FX offline on the drain, which is also
+        // where the consumed snapshot is dropped.
+        for (const TrackPlan& tp : plan.restore) {
+            if (tp.flags.empty()) continue;
+            const std::string& guid = tp.flags.front().guid;
+            MediaTrack* tr = resolve(trackByGuid, guid);
+            if (!tr) continue;  // stale GUID — prune
+
+            applyFlags(tr, tp.flags, undoMask);
+            deferFxRestore(proj, guid, tp.fxOffline);
+        }
+
+        // MANAGED LANES: drive C_LANEPLAYS so the active mode's lane plays+shows
+        // and every other managed lane is silenced+hidden. Empty for a D1-only
+        // project, leaving that behavior byte-identical.
+        laneModeChanged = applyLaneOps(trackByGuid, plan.lanes);
+
+        // PARENT VISIBILITY (never parked): recomputed every toggle, never
+        // snapshotted. Only the two visibility flags — never mainSend/FX on a parent.
+        std::set<std::string> visible = model.visibleTracks(tree, targetModeId);
+        for (const FolderNode& node : tree.nodes) {
+            if (!node.isParent) continue;
+            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);
+            writeIfChanged(tr, "B_SHOWINMIXER", show);
+        }
+
+        model.setActiveMode(targetModeId);
+
+        // Replay + consume, before the single relayout below picks the change up.
+        // Dropped against `visible` (computed above for parent visibility), not the
+        // park plan: a folder parent can be hidden without being parked, and a
+        // hidden track must not receive a replayed solo it carries no visible
+        // control to undo.
+        if (realSwitch) {
+            if (const std::map<std::string, int>* cached = model.soloCache().query(targetModeId)) {
+                restoreTrackSolos(handleByGuid, *cached, liveGuids, visible);
+                model.soloCache().clear(targetModeId);
+            }
         }
     }
 
@@ -560,12 +539,12 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
     // fixed lanes this apply (SDK requirement for I_FREEMODE changes).
     if (laneModeChanged) UpdateTimeline();
 
-    Undo_EndBlock2(proj, undoLabel.c_str(), -1);
+    Undo_EndBlock2(proj, undoLabel.c_str(), undoMask);
     return true;
 }
 
 bool mintManagedLanes(ViewModeModel& model, ReaProject* proj) {
-    std::vector<std::pair<std::string, MediaTrack*>> handleByGuid;
+    TrackHandles handleByGuid;
     std::vector<TrackFolderEntry> entries = readFolderEntries(proj, handleByGuid);
     // The tree is needed to detect a content-bearing folder derived-visible in
     // >1 mode, exactly as applyMode builds it.
@@ -575,8 +554,10 @@ bool mintManagedLanes(ViewModeModel& model, ReaProject* proj) {
     const LaneMintPlan plan = planLaneMinting(model, tree, tracks);
     if (plan.empty()) return false;  // nothing to mint — no Undo point for a no-op tick
 
+    const TrackByGuid trackByGuid = indexHandles(handleByGuid);
+
     Undo_BeginBlock2(proj);
-    const bool changed = applyMintPlan(model, plan, handleByGuid);
+    const bool changed = applyMintPlan(model, plan, trackByGuid);
 
     if (!changed) {
         // Plan was non-empty but every write was already satisfied — discard
@@ -599,17 +580,19 @@ bool mintManagedLanes(ViewModeModel& model, ReaProject* proj) {
     // here — it would re-park/restore whole tracks and recompute parent
     // visibility, which a lane-only mint must not touch.
     const TogglePlan togglePlan = model.planToggle(FolderTree{}, model.activeModeId());
-    applyLaneOps(handleByGuid, togglePlan.lanes);
+    applyLaneOps(trackByGuid, togglePlan.lanes);
 
     UpdateTimeline();  // a split happened this call — refresh is owed
     UpdateArrange();
 
-    Undo_EndBlock2(proj, "ReaSampler: separate cross-mode content into lanes", -1);
+    // Lane minting writes track lane config and item lane assignment and nothing
+    // else — never an FX state — so it takes the same honest mask applyMode does.
+    Undo_EndBlock2(proj, "ReaSampler: separate cross-mode content into lanes", kApplyUndoMask);
     return true;
 }
 
 void reconcileManagedLanes(ViewModeModel& model, ReaProject* proj) {
-    std::vector<std::pair<std::string, MediaTrack*>> handleByGuid;
+    TrackHandles handleByGuid;
     readFolderEntries(proj, handleByGuid);  // populates handleByGuid (tree unused here)
 
     // Pure read of REAPER state (no lane created, no I_FREEMODE/I_NUMFIXEDLANES/

src/shell/view/view_fx_park.cpp

@@ -0,0 +1,157 @@
+// See view_fx_park.h. Compiled into the reaper_reasampler module; includes
+// reaper_plugin_functions.h without REAPERAPI_IMPLEMENT (main.cpp owns that).
+// The queue's rule is pure (header, test_view_fx_park.cpp); this file owns the
+// REAPER reads/writes and the ordering against the live enumeration.
+
+#include "shell/view/view_fx_park.h"
+
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "core/view/view_mode_model.h"
+#include "shell/capture/track_guid.h"
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_CountTracks
+#define REAPERAPI_WANT_EnumProjects
+#define REAPERAPI_WANT_GetTrack
+#define REAPERAPI_WANT_ShowConsoleMsg
+#define REAPERAPI_WANT_TrackFX_GetCount
+#define REAPERAPI_WANT_TrackFX_GetFXGUID
+#define REAPERAPI_WANT_TrackFX_GetOffline
+#define REAPERAPI_WANT_TrackFX_SetOffline
+#define REAPERAPI_WANT_guidToString
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+
+FxParkQueue g_queue;
+ReaProject* g_owner = nullptr;  // the project the pending intents were enqueued against
+
+ReaProject* currentProject() { return EnumProjects(-1, nullptr, 0); }
+
+void adoptOwner(ReaProject* proj) {
+    ReaProject* p = proj ? proj : currentProject();
+    if (p == g_owner) return;
+    g_queue.clear();  // intents planned against another project are never replayed here
+    g_owner = p;
+}
+
+// TrackFX_SetOffline unloads and re-instantiates the plugin, so writing a state
+// that already holds is not a no-op — it is the expensive half of a mode switch
+// spent on nothing.
+void setOfflineIfChanged(MediaTrack* tr, int fx, bool offline) {
+    if (TrackFX_GetOffline(tr, fx) == offline) return;
+    TrackFX_SetOffline(tr, fx, offline);
+}
+
+void applyPark(MediaTrack* tr) {
+    const int fxCount = TrackFX_GetCount(tr);
+    for (int fx = 0; fx < fxCount; ++fx) setOfflineIfChanged(tr, fx, true);
+}
+
+// Restores per-FX offline from the plan verbatim — never a blanket "online".
+// Which live FX each captured state belongs to is resolveFxRestore's call, and
+// what it could not place comes back for the caller to report.
+FxRestoreDrops applyRestore(MediaTrack* tr, const std::vector<FxOfflineOp>& ops) {
+    const FxRestoreResolution res = resolveFxRestore(ops, liveFxGuids(tr));
+    for (const FxOfflineWrite& w : res.writes) setOfflineIfChanged(tr, w.fxIndex, w.offline);
+    return res.drops;
+}
+
+// The FX's own durable identity, braced exactly like the track GUID keys. Empty
+// when REAPER reports none — an FX we cannot name is one we cannot restore, and
+// fx_offline treats it that way rather than guessing at its slot. Lifetime is
+// settled: the string copy is taken immediately and the GUID* is never held
+// past this call (reaper_plugin_functions.h:7348 documents no null contract for
+// TrackFX_GetFXGUID; treating null as "no identity" is the safe read).
+//
+// [verify — DAW] STABILITY across a chain mutation is not settled the same way:
+// confirm the GUID for one FX instance survives a native drag-reorder, an SWS
+// move (SNM_MoveOrRemoveTrackFX — SWS issue #802 reports the FXID lines do not
+// follow the plugin after that call, i.e. wrong-plugin restores or mass drops
+// through fx_offline on that path specifically), a save/reload round trip, and
+// two live instances of one plugin type staying distinguishable. See
+// src/shell/view/CLAUDE.md's Gotchas for the SWS-path risk this leaves open.
+std::string fxGuidString(MediaTrack* tr, int fx) {
+    GUID* g = TrackFX_GetFXGUID(tr, fx);
+    if (!g) return {};
+    char buf[64] = {0};  // guidToString needs a >=64-char destination (SDK contract)
+    guidToString(g, buf);
+    return std::string(buf);
+}
+
+}  // namespace
+
+std::vector<std::string> liveFxGuids(MediaTrack* tr) {
+    const int fxCount = TrackFX_GetCount(tr);
+    std::vector<std::string> guids;
+    guids.reserve(static_cast<std::size_t>(fxCount));
+    for (int fx = 0; fx < fxCount; ++fx) guids.push_back(fxGuidString(tr, fx));
+    return guids;
+}
+
+void deferFxPark(ReaProject* proj, const std::string& guid) {
+    adoptOwner(proj);
+    g_queue.park(guid);
+}
+
+void deferFxRestore(ReaProject* proj, const std::string& guid, std::vector<FxOfflineOp> ops) {
+    adoptOwner(proj);
+    g_queue.restore(guid, std::move(ops));
+}
+
+void drainDeferredFxParks(ViewModeModel& model) {
+    if (g_queue.empty()) return;
+
+    if (g_owner != currentProject()) {
+        // The project the intents were planned against was closed or switched
+        // away from: its tracks are not ours to write and its handles may be
+        // gone. Discard rather than apply.
+        g_queue.clear();
+        g_owner = nullptr;
+        return;
+    }
+
+    std::unordered_map<std::string, MediaTrack*> byGuid;
+    const int count = CountTracks(g_owner);
+    byGuid.reserve(static_cast<std::size_t>(count));
+    for (int i = 0; i < count; ++i) {
+        MediaTrack* tr = GetTrack(g_owner, i);
+        if (!tr) continue;
+        std::string guid = guidString(tr);
+        if (!guid.empty()) byGuid.emplace(std::move(guid), tr);
+    }
+
+    FxRestoreDrops drops;
+    int dropTracks = 0;
+    for (const FxParkIntent& intent : g_queue.pending()) {
+        auto it = byGuid.find(intent.guid);
+        if (it == byGuid.end()) continue;  // track deleted since the switch — prune
+        if (intent.park) {
+            applyPark(it->second);
+            continue;
+        }
+        const FxRestoreDrops d = applyRestore(it->second, intent.restoreOps);
+        if (d.total() > 0) {
+            drops.add(d);
+            ++dropTracks;
+        }
+        model.clearSnapshot(intent.guid);
+    }
+
+    g_queue.clear();
+
+    // Captured FX state that could not be applied is REPORTED — silence would read
+    // to the user as "restore worked" while an FX sat at whatever state the park
+    // left it in. The "!SHOW:" prefix (reaper_plugin_functions.h:6536) keeps it from
+    // force-opening the console: this drain also runs behind an unattended
+    // project-load reapply, and cannot tell that case from an interactive toggle.
+    const std::string fxDropMsg = describeFxRestoreDrops(drops, dropTracks);
+    if (!fxDropMsg.empty()) ShowConsoleMsg(("!SHOW:" + fxDropMsg).c_str());
+}
+
+}  // namespace reasampler

src/shell/view/view_fx_park.h

@@ -0,0 +1,95 @@
+#pragma once
+// Design View's per-FX offline surface: the FX-identity read that snapshot,
+// park and restore all address FX through, the deferred intent queue that keeps
+// TrackFX_SetOffline off the mode switch's synchronous path, and the idle-tick
+// drain that applies it. See src/shell/view/CLAUDE.md's FX-parking caveat.
+
+#include <string>
+#include <vector>
+
+#include "core/view/fx_offline.h"
+
+// Forward-declared to keep this header SDK-free; the .cpp includes the real SDK header.
+class MediaTrack;
+class ReaProject;
+
+namespace reasampler {
+
+class ViewModeModel;
+
+// The chain as it stands now: identity by current slot. Snapshot, park and
+// restore all address FX through this one plain 0..TrackFX_GetCount-1
+// enumeration — never the 0x1000000/0x2000000 input-FX or container forms — so
+// whatever it covers, all three cover identically.
+std::vector<std::string> liveFxGuids(MediaTrack* tr);
+
+// One deferred per-FX intent for one track. A park carries no ops (every live
+// slot goes offline); a restore carries the planned ops verbatim.
+struct FxParkIntent {
+    std::string              guid;
+    bool                     park = false;
+    std::vector<FxOfflineOp> restoreOps;
+};
+
+// The queue's re-entrancy rule, pure so it can be asserted without a DAW: at
+// most ONE intent per track GUID, and the latest one wins. Park and restore are
+// inverses, so an intent landing on its own pending inverse CANCELS it rather
+// than stacking — the queued work never ran, so the track already holds the
+// state the newcomer asks for, and replaying both would be both slower and
+// observably wrong.
+class FxParkQueue {
+public:
+    void park(const std::string& guid) {
+        FxParkIntent* held = find(guid);
+        if (!held) {
+            pending_.push_back(FxParkIntent{guid, true, {}});
+        } else if (!held->park) {
+            erase(held);
+        }
+    }
+
+    void restore(const std::string& guid, std::vector<FxOfflineOp> ops) {
+        FxParkIntent* held = find(guid);
+        if (!held) {
+            pending_.push_back(FxParkIntent{guid, false, std::move(ops)});
+        } else if (held->park) {
+            erase(held);
+        } else {
+            held->restoreOps = std::move(ops);
+        }
+    }
+
+    // Enqueue order, which is apply order: park before restore within one
+    // switch, as the synchronous body already orders them.
+    const std::vector<FxParkIntent>& pending() const { return pending_; }
+    bool empty() const { return pending_.empty(); }
+    void clear() { pending_.clear(); }
+
+private:
+    FxParkIntent* find(const std::string& guid) {
+        for (FxParkIntent& i : pending_)
+            if (i.guid == guid) return &i;
+        return nullptr;
+    }
+    void erase(FxParkIntent* held) {
+        pending_.erase(pending_.begin() + (held - pending_.data()));
+    }
+
+    std::vector<FxParkIntent> pending_;
+};
+
+// Enqueue against `proj` (nullptr = current project). An enqueue naming a
+// different project than the pending intents discards those unapplied.
+void deferFxPark(ReaProject* proj, const std::string& guid);
+void deferFxRestore(ReaProject* proj, const std::string& guid, std::vector<FxOfflineOp> ops);
+
+// Applies every pending intent, then clears each drained restore's snapshot
+// from `model` — the snapshot is consumed when the restore actually lands, not
+// when it was planned, so a second switch arriving first re-parks against the
+// still-true captured state instead of re-capturing parked values. Discards the
+// queue unapplied if the project it was enqueued against is no longer current
+// (close / switch); an intent whose track is gone is pruned. Idle cost is one
+// empty-queue test.
+void drainDeferredFxParks(ViewModeModel& model);
+
+} // namespace reasampler