Refuse to park a track whose pre-park snapshot is gone, rather than re-snapshotting the already-parked chain as the user's state

src/shell/view/CLAUDE.md

@@ -30,6 +30,15 @@ decide membership or mode rules.
   (hide both panels), `B_MAINSEND=0` (out of mix), `I_FXEN=0` (FX bypassed), and
   `TrackFX_SetOffline(track, fx, true)` for each FX (reclaim CPU) — full CPU-park,
   not mix-removal-only.
+- **A snapshot is only ever taken from a chain no park has touched.** An absent
+  snapshot is NOT evidence of a clean chain — `discardDeferredFxParks` drops
+  intents whose flag writes already landed, so a reloaded model and an
+  already-parked project routinely coexist. A park meeting that pair REFUSES:
+  no flag write, no FX-offline enqueue, no snapshot, and one console line naming
+  the hand recovery. Leaving the track as found is the only non-destructive
+  answer once the pre-park truth is gone — snapshotting there would record park
+  state as the user's state and every later restore would faithfully write it
+  back. Decision at `view_fx_park`'s `decidePark`.
 - **Non-destructive restore.** For every flag the tool drives, snapshot the prior
   value BEFORE parking; on toggle-back restore FROM the snapshot, never to a
   hardcoded "on." Round-trip (snapshot → park → restore) returns every driven flag
@@ -107,7 +116,7 @@ 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`, 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 owns no model state.** A snapshot is dropped where the restore is PLANNED — the flags are back at their captured values from that moment, and a model that still described the track as parked would let a persist or a reapply inside the drain window replan a restore over whatever the user changed since. What the deferral costs instead is that the live FX chain stops being a trustworthy snapshot source while an intent is pending: a park that CANCELS a pending restore takes the pre-park FX states from that restore's ops (`preParkFxFromCancelledRestore`), because the chain still reads the parked values and the cancel means no drain will ever fix them.
+- `view_fx_park` — the park 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), the idle-tick drain `main.cpp`'s `OnTimer` calls, and the pure per-track park decision (`decidePark` over "does the model hold a snapshot" × "does the chain already read parked", plus `trackFlagParm` — the ONE `Flag` → REAPER-parameter mapping park's live read and restore's write both address). **The drain owns no model state.** A snapshot is dropped where the restore is PLANNED — the flags are back at their captured values from that moment, and a model that still described the track as parked would let a persist or a reapply inside the drain window replan a restore over whatever the user changed since. What the deferral costs instead is that the live FX chain stops being a trustworthy snapshot source while an intent is pending: a park that CANCELS a pending restore takes the pre-park FX states from that restore's ops (`preParkFxFromCancelledRestore`), because the chain still reads the parked values and the cancel means no drain will ever fix them.
 - `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
@@ -130,7 +139,9 @@ applies the resulting lane state to live tracks.
   over the reloaded model, re-planning a park for every inactive leaf, so parked
   FX converge on the following drain. The one case that does not self-heal is a
   track whose reloaded model carries no snapshot — nothing plans its restore, so
-  FX left offline stay offline. Full contract at `discardDeferredFxParks`.
+  FX left offline stay offline, and its next park REFUSES rather than
+  re-snapshotting the parked chain (snapshot-source invariant above). Stuck, but
+  never falsely committed. Full contract at `discardDeferredFxParks`.
 - `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_fx_park.cpp`) and SWS issue #802 is a

src/shell/view/view.cpp

@@ -31,6 +31,7 @@
 #define REAPERAPI_WANT_SetMediaTrackInfo_Value
 #define REAPERAPI_WANT_GetSetMediaTrackInfo_String
 #define REAPERAPI_WANT_PreventUIRefresh
+#define REAPERAPI_WANT_ShowConsoleMsg
 #define REAPERAPI_WANT_Undo_BeginBlock2
 #define REAPERAPI_WANT_Undo_EndBlock2
 #define REAPERAPI_WANT_TrackList_AdjustWindows
@@ -107,16 +108,6 @@ void applyTransparentLaneDisplay(MediaTrack* tr) {
                             static_cast<double>(settings | kLaneSettingsHideButtons));
 }
 
-const char* flagParm(Flag f) {
-    switch (f) {
-        case Flag::ShowInTcp:   return "B_SHOWINTCP";
-        case Flag::ShowInMixer: return "B_SHOWINMIXER";
-        case Flag::MainSend:    return "B_MAINSEND";
-        case Flag::FxEnable:    return "I_FXEN";
-    }
-    return "B_SHOWINTCP";  // unreachable; keeps the compiler quiet
-}
-
 // The master track is not enumerated by GetTrack (index space excludes it),
 // so it can never enter the tree — the master-untouched invariant holds by
 // construction.
@@ -185,7 +176,7 @@ bool writeIfChanged(MediaTrack* tr, const char* parm, double value) {
 
 void applyFlags(MediaTrack* tr, const std::vector<TrackFlagOp>& flags, int& undoMask) {
     for (const TrackFlagOp& op : flags) {
-        if (!writeIfChanged(tr, flagParm(op.flag), static_cast<double>(op.value))) continue;
+        if (!writeIfChanged(tr, trackFlagParm(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;
@@ -449,6 +440,7 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
 
     int undoMask = kApplyUndoMask;
     bool laneModeChanged = false;
+    int refusedParks = 0;
 
     Undo_BeginBlock2(proj);
     {
@@ -470,13 +462,22 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
             MediaTrack* tr = resolve(trackByGuid, guid);
             if (!tr) continue;  // stale GUID — prune
 
+            // Full contract at decidePark. Only an absent snapshot can refuse, so
+            // the short-circuit cannot change the answer (ParkOnly ignores the
+            // second term); a first park still pays the four reads, a reapply's
+            // re-park of a still-parked track does not.
+            const bool haveSnapshot = model.snapshot(guid) != nullptr;
+            const ParkAction action = decidePark(
+                haveSnapshot,
+                !haveSnapshot &&
+                    parkFlagsAlreadyApplied(tp.flags, readParkFlagValues(tr, tp.flags)));
+            if (action == ParkAction::Refuse) { ++refusedParks; continue; }
+
             // Enqueued FIRST: a park landing on this track's own pending restore
             // cancels it, and those ops are then the only surviving record of the
-            // pre-park FX state. Snapshot ONCE — a snapshot already present means
-            // the track is still parked, so recapturing would overwrite the true
-            // pre-park state with zeros and a later restore would hide it for good.
+            // pre-park FX state.
             const std::vector<FxOfflineOp> cancelled = deferFxPark(proj, guid);
-            if (model.snapshot(guid) == nullptr)
+            if (action == ParkAction::SnapshotThenPark)
                 model.storeSnapshot(guid, snapshotTrack(tr, cancelled));
             applyFlags(tr, tp.flags, undoMask);
         }
@@ -539,6 +540,13 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
     if (laneModeChanged) UpdateTimeline();
 
     Undo_EndBlock2(proj, undoLabel.c_str(), undoMask);
+
+    // After the undo block and the UI hold: a refusal is a report, not a project
+    // write, and it must not join what a Ctrl-Z rolls back. "!SHOW:" so an
+    // unattended project-load reapply cannot force the console open
+    // (reaper_plugin_functions.h:6536), same as the drain's drop report.
+    const std::string refusedMsg = describeRefusedParks(refusedParks);
+    if (!refusedMsg.empty()) ShowConsoleMsg(("!SHOW:" + refusedMsg).c_str());
     return true;
 }
 

src/shell/view/view_fx_park.cpp

@@ -14,6 +14,7 @@
 #define REAPERAPI_MINIMAL
 #define REAPERAPI_WANT_CountTracks
 #define REAPERAPI_WANT_EnumProjects
+#define REAPERAPI_WANT_GetMediaTrackInfo_Value
 #define REAPERAPI_WANT_GetTrack
 #define REAPERAPI_WANT_ShowConsoleMsg
 #define REAPERAPI_WANT_TrackFX_GetCount
@@ -99,6 +100,14 @@ std::string fxGuidString(MediaTrack* tr, int fx) {
 
 }  // namespace
 
+std::vector<int> readParkFlagValues(MediaTrack* tr, const std::vector<TrackFlagOp>& parkFlags) {
+    std::vector<int> live;
+    live.reserve(parkFlags.size());
+    for (const TrackFlagOp& op : parkFlags)
+        live.push_back(static_cast<int>(GetMediaTrackInfo_Value(tr, trackFlagParm(op.flag))));
+    return live;
+}
+
 std::vector<std::string> liveFxGuids(MediaTrack* tr) {
     const int fxCount = TrackFX_GetCount(tr);
     std::vector<std::string> guids;

src/shell/view/view_fx_park.h

@@ -1,13 +1,16 @@
 #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.
+// Design View's park 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, the idle-tick drain
+// that applies it, and the trust test deciding whether a chain may be
+// snapshotted at all. See src/shell/view/CLAUDE.md's FX-parking caveat.
 
+#include <cstddef>
 #include <string>
 #include <vector>
 
 #include "core/view/fx_offline.h"
+#include "core/view/view_mode_model.h"  // Flag / TrackFlagOp — what a park writes
 
 // Forward-declared to keep this header SDK-free; the .cpp includes the real SDK header.
 class MediaTrack;
@@ -103,6 +106,70 @@ private:
     std::vector<FxParkIntent> pending_;
 };
 
+// -- may this chain be snapshotted? -------------------------------------------
+//
+// A pre-park snapshot may only ever be taken from a chain NO park has touched.
+// Its values are restore's sole source of truth, so a snapshot lifted from an
+// already-parked chain records park state as the user's state, and every later
+// restore then faithfully writes hidden/out-of-mix/FX-disabled back — permanent
+// and silent. A model carrying no snapshot is NOT evidence the chain is clean:
+// discardDeferredFxParks drops intents whose flag writes already landed, so a
+// reloaded model and an already-parked project routinely coexist.
+
+// The REAPER track parameter each driven flag addresses. One home for the
+// mapping — park's live read, restore's write and the trust test below must
+// never disagree about which parameter a Flag names.
+inline const char* trackFlagParm(Flag f) {
+    switch (f) {
+        case Flag::ShowInTcp:   return "B_SHOWINTCP";
+        case Flag::ShowInMixer: return "B_SHOWINMIXER";
+        case Flag::MainSend:    return "B_MAINSEND";
+        case Flag::FxEnable:    return "I_FXEN";
+    }
+    return "B_SHOWINTCP";  // unreachable; keeps the compiler quiet
+}
+
+// The live values of exactly the flags `parkFlags` would write, in that order.
+std::vector<int> readParkFlagValues(MediaTrack* tr, const std::vector<TrackFlagOp>& parkFlags);
+
+// Does the track already sit at every value this park would write? Then a park
+// reached it. An empty plan is evidence of nothing and answers false.
+inline bool parkFlagsAlreadyApplied(const std::vector<TrackFlagOp>& parkFlags,
+                                    const std::vector<int>& liveValues) {
+    if (parkFlags.empty() || parkFlags.size() != liveValues.size()) return false;
+    for (std::size_t i = 0; i < parkFlags.size(); ++i)
+        if (liveValues[i] != parkFlags[i].value) return false;
+    return true;
+}
+
+enum class ParkAction {
+    SnapshotThenPark,  // clean chain — capture the pre-park truth, then park
+    ParkOnly,          // still parked; the held snapshot IS the pre-park truth
+    Refuse,            // no snapshot AND a parked chain — the truth is gone
+};
+
+// Refusing leaves the track exactly as found: no flag write, no FX-offline
+// enqueue, no snapshot. Nothing false is committed, so the moment the user's own
+// edit puts the chain back somewhere trustworthy the next park captures it
+// correctly — where a snapshot of the parked state would have closed that door
+// for good.
+inline ParkAction decidePark(bool haveSnapshot, bool chainReadsParked) {
+    if (haveSnapshot) return ParkAction::ParkOnly;
+    return chainReadsParked ? ParkAction::Refuse : ParkAction::SnapshotThenPark;
+}
+
+// One console line for the tracks an apply refused, empty when none. Names the
+// hand recovery: nothing else will bring a refused track back.
+inline std::string describeRefusedParks(int trackCount) {
+    if (trackCount <= 0) return {};
+    return "ReaSampler: " + std::to_string(trackCount) + " track" +
+           (trackCount == 1 ? "" : "s") +
+           " left unparked — the pre-park state was lost (an undo/redo or a project "
+           "reload dropped it), and parking now would record the parked state as yours. "
+           "Restore each track's TCP/mixer visibility, main send and FX enable by hand; "
+           "the next mode switch will then park it correctly.\n";
+}
+
 // The FX half of a snapshot, with the keying it must be read back under.
 struct PreParkFx {
     std::vector<FxOfflineState> states;