fix(view): derive parent/folder visibility on mode toggle

applyMode now drives B_SHOWINTCP/B_SHOWINMIXER for every parent node
from visibleTracks(tree, targetMode); parents are never parked. Adds
restoreFxOffline by-index remap hazard note.

src/view.cpp

@@ -10,6 +10,7 @@
 
 #include "view.h"
 
+#include <set>
 #include <string>
 #include <vector>
 
@@ -127,6 +128,12 @@ void parkFxOffline(MediaTrack* tr) {
 // Restores per-FX offline from the snapshot verbatim — each slot back to its
 // captured value, never a blanket "online". Bounds-checked against the live FX
 // count in case the plugin chain changed while parked (prune-safe).
+//
+// HAZARD (deferred, PLAN "reconcile on delete/restructure"): the remap is by
+// slot INDEX, not plugin identity. If the FX chain changed while the track was
+// parked, snapshot slot k is restored onto whatever plugin now occupies slot k —
+// the bounds-check guards against out-of-range, not against a reshuffled chain.
+// Acceptable for D2; full identity-based reconciliation is future hardening.
 void restoreFxOffline(MediaTrack* tr, const std::vector<FxOfflineOp>& fxOffline) {
     int fxCount = TrackFX_GetCount(tr);
     for (const FxOfflineOp& op : fxOffline) {
@@ -179,6 +186,21 @@ bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject
         model.clearSnapshot(guid);
     }
 
+    // PARENT VISIBILITY (derived, never parked): a folder is visible iff at least
+    // one of its descendant leaves is visible in the target mode. That derivation is
+    // pure (membership + active mode), so it is recomputed every toggle rather than
+    // snapshotted — the four leaf-park flags don't apply to parents. Drive only the
+    // two visibility flags; never touch B_MAINSEND/I_FXEN/FX-offline 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);
+    }
+
     model.setActiveMode(targetModeId);
 
     Undo_EndBlock2(proj, "ReaSampler: apply Design View mode", -1);