Fix drag-handoff bugs: gate FX re-resolve on outside-panel, cache unresolvable OS-drag verdict, block double FX-add retry

src/shell/actions/instrument_drop_win.cpp

@@ -104,16 +104,23 @@ bool loadInstrumentOntoTrack(MediaTrack* track, const std::vector<std::uint8_t>&
     const std::string fxName = "VST3:" + vstPluginName();
 
     // An EXPLICIT top-level insertion position (instantiate <= -1000 IS the position, -1000
-    // = first in chain), not the bare -1. Both always create a new instance; the bare form
-    // additionally leaves placement to REAPER's ambient FX-chain insert point, which a drop
-    // onto an FX container/chain-window moves — so the index handed to TrackFX_SetPreset and
-    // the instance just created stop denoting the same FX and the capture never lands. The
-    // bare-form retry keeps the reference path alive if the positional form is ever refused.
+    // = first in chain), not the bare -1 — this form is documented in the SDK header. Both
+    // always create a new instance; the bare form additionally leaves placement to REAPER's
+    // ambient FX-chain insert point, which a drop onto an FX container/chain-window is
+    // suspected (unconfirmed by experiment) to move — if so, the index handed to
+    // TrackFX_SetPreset and the instance just created would stop denoting the same FX and the
+    // capture would never land. The bare-form retry keeps the reference path alive if the
+    // positional form is ever refused.
     // recFX = false: a normal track FX chain instance, not a record/monitoring FX.
     const int insertPos = TrackFX_GetCount(track);
     int fxIndex = TrackFX_AddByName(track, fxName.c_str(), /*recFX=*/false,
                                     /*instantiate=*/-1000 - insertPos);
-    if (fxIndex < 0)
+    // Only retry with the bare form when the chain is PROVABLY unchanged (count still
+    // insertPos): a negative return with the count grown means the positional add DID create
+    // an instance and just reported -1 — retrying then would add a SECOND instance, leaving
+    // the first orphaned (no preset applied, unreachable for rollback), which is exactly the
+    // all-or-nothing violation this contract forbids.
+    if (fxIndex < 0 && TrackFX_GetCount(track) == insertPos)
         fxIndex = TrackFX_AddByName(track, fxName.c_str(), /*recFX=*/false, /*instantiate=*/-1);
 
     // u8string() gives UTF-8 bytes on MSVC (not ACP-converted), so a temp dir under