fix(drop): recognize TCP/MCP FX-button hotspot for instrument drop (S-VIEW-BUG-1)

The FX-hotspot classifier matched only fx_ (the FX-chain/floating windows),
so a drop on a track TCP FX button (info tcp*) never armed the drop and the
capture fell through to arrange as audio. Widen to fx_/tcp/mcp in a pure,
unit-tested predicate; shell calls it.

src/instrument_drop.cpp

@@ -39,6 +39,14 @@ std::string buildInstrumentDropChunk(const std::string& sampleId) {
     return encodeBase64(instrumentDropStateBytes(sampleId));
 }
 
+bool infoNamesFxHotspot(const std::string& info) {
+    // See the header contract. The FX-chain / floating-FX windows report "fx_*"; the track /
+    // mixer panel that carries the FX button reports "tcp*" / "mcp*". Prefix-match all three —
+    // the TCP/MCP surfaces are the fix for S-VIEW-BUG-1 (the "fx_"-only predicate missed them).
+    auto startsWith = [&info](const char* p) { return info.rfind(p, 0) == 0; };
+    return startsWith("fx_") || startsWith("tcp") || startsWith("mcp");
+}
+
 std::string encodeBase64(const std::vector<std::uint8_t>& bytes) {
     std::string out;
     out.reserve(((bytes.size() + 2) / 3) * 4);

src/instrument_drop.h

@@ -49,6 +49,26 @@ namespace reasampler {
 // Deterministic: the same sampleId always yields the same blob (base64 of the same bytes).
 std::string buildInstrumentDropChunk(const std::string& sampleId);
 
+// -- FX-drop-target classification (S-VIEW-BUG-1) ------------------------------
+//
+// Pure classifier for GetThingFromPoint's info string: is the point over a surface where an
+// instrument drop should instantiate ReaSampler 9000 on the resolved track? This is string
+// logic (no REAPER types), so it lives here and is unit-tested outside the DAW — the shell
+// (instrument_drop_win) only supplies the info bytes GetThingFromPoint filled.
+//
+// THE BUG (S-VIEW-BUG-1): the original shell predicate matched ONLY "fx_" — but per the SDK
+// (reaper_plugin_functions.h §GetThingFromPoint) "fx_chain"/"fx_N" are the FX-CHAIN and
+// FLOATING-FX windows; a hit on the TRACK PANEL (where the TCP/MCP FX button actually lives —
+// the intuitive "drop onto the track's FX chain" target) reports a string that BEGINS WITH
+// "tcp" or "mcp" (e.g. "tcp.fx"). So dropping on the TCP FX button never armed the drop; the
+// gesture fell through and the file dropped to arrange as audio. The drop target is therefore
+// EITHER of the SDK's two documented FX-bearing surfaces:
+//   * the FX chain / floating FX window  -> info begins with "fx_" ("fx_chain", "fx_0", ...)
+//   * the track/mixer panel that hosts the FX button -> info begins with "tcp" or "mcp"
+// A single-capture drag released over any of these, on a resolved track, is an instrument drop.
+// Any other info ("arrange", "spacer_0", "", ...) is not an FX hotspot.
+bool infoNamesFxHotspot(const std::string& info);
+
 // The raw (pre-base64) component-state bytes — exposed so the round-trip test can decode them
 // back through the instrument's OWN reader (sample_map::deserializeComponentState) and assert
 // the capture is selected, proving buildInstrumentDropChunk feeds the instrument exactly what

src/instrument_drop_win.cpp

@@ -5,10 +5,10 @@
 
 #include "instrument_drop_win.h"
 
-#include <cstring>
 #include <string>
 
-#include "app_version.h"  // vstPluginName() — the CHANNEL-correct FX name (stable/beta pairing)
+#include "app_version.h"      // vstPluginName() — the CHANNEL-correct FX name (stable/beta pairing)
+#include "instrument_drop.h"  // infoNamesFxHotspot — the PURE, unit-tested hotspot classifier
 
 #include "reaper_plugin.h"
 
@@ -23,19 +23,6 @@
 
 namespace reasampler {
 
-namespace {
-
-// GetThingFromPoint's info string prefixes (verified against reaper_plugin_functions.h:
-// "Updates infoOut with information such as 'arrange', 'fx_chain', 'fx_0' ... If a track
-// panel is hit, string will begin with 'tcp' or 'mcp' or 'tcp.mute' etc"). The FX region
-// reports "fx_chain" (the FX list area) or "fx_N" (a specific FX button). We treat either
-// as the FX hotspot — the S17 drop target.
-bool infoNamesFxHotspot(const char* info) {
-    return std::strncmp(info, "fx_", 3) == 0;
-}
-
-}  // namespace
-
 FxDropTarget resolveFxDropTarget(int screenX, int screenY) {
     FxDropTarget out;
     char info[256] = {0};
@@ -46,6 +33,8 @@ FxDropTarget resolveFxDropTarget(int screenX, int screenY) {
     MediaTrack* track = GetThingFromPoint(screenX, screenY, info, sizeof(info));
     out.track = track;
     out.overReaperUi = (track != nullptr) || (info[0] != '\0');
+    // S-VIEW-BUG-1: the hotspot is either the FX chain/floating window ("fx_*") OR the track/
+    // mixer panel that hosts the FX button ("tcp*"/"mcp*"). The pure classifier owns the rule.
     out.overFxHotspot = (track != nullptr) && infoNamesFxHotspot(info);
     return out;
 }

tests/test_instrument_drop.cpp

@@ -105,6 +105,44 @@ static void testBase64DecodeRejectsMalformed() {
     CHECK(decodeBase64("Zg==Zg==").empty()); // interior padding (pad before the final quad)
 }
 
+// --- FX-hotspot classification (S-VIEW-BUG-1) ---------------------------------
+//
+// THE BUG: dropping a capture onto a track's FX button (in the TCP) never armed the
+// instrument drop, because the old predicate matched only "fx_" — which the SDK reserves for
+// the FX CHAIN / FLOATING-FX windows. A track-panel hit (where the FX button lives) reports a
+// string beginning with "tcp"/"mcp". These tests fix the classifier at the boundary the shell
+// consumes: each would FAIL under the old "fx_"-only rule for the tcp/mcp cases.
+
+// The regression case: the TCP FX-button surface reports "tcp*" and MUST classify as an FX
+// hotspot. Under the pre-fix "fx_"-only predicate these all returned false — the exact miss
+// that produced the "drops as audio to arrange" symptom.
+static void testTcpMcpPanelIsHotspot() {
+    CHECK(infoNamesFxHotspot("tcp"));       // bare track control panel
+    CHECK(infoNamesFxHotspot("tcp.fx"));    // the TCP FX-button WALTER element
+    CHECK(infoNamesFxHotspot("tcp.mute"));  // any tcp.* sub-element resolves to the track
+    CHECK(infoNamesFxHotspot("mcp"));       // mixer control panel
+    CHECK(infoNamesFxHotspot("mcp.fx"));    // the MCP FX area
+}
+
+// The FX chain / floating-FX windows (the surfaces the ORIGINAL predicate matched) still
+// classify as hotspots — the fix widens the rule, it does not regress the "fx_" surface.
+static void testFxWindowStillHotspot() {
+    CHECK(infoNamesFxHotspot("fx_chain"));  // FX chain window
+    CHECK(infoNamesFxHotspot("fx_0"));      // first FX, floating
+    CHECK(infoNamesFxHotspot("fx_12"));     // arbitrary floating-FX index
+}
+
+// Non-FX surfaces are NOT hotspots — a drop here is not an instrument drop (it would fall
+// through to the OS drag / no-op). Guards against over-broad matching (e.g. "spacer_0" must
+// not match despite living near the tracks; "arrange" is the audio-import surface).
+static void testNonFxSurfacesAreNotHotspot() {
+    CHECK(!infoNamesFxHotspot("arrange"));
+    CHECK(!infoNamesFxHotspot("spacer_0"));
+    CHECK(!infoNamesFxHotspot(""));          // pointer over nothing REAPER classifies
+    CHECK(!infoNamesFxHotspot("trans"));     // transport
+    CHECK(!infoNamesFxHotspot("envcp"));     // envelope control panel — a track thing, not FX
+}
+
 int main() {
     testBlobRoundTripsThroughInstrumentReader();
     testGuidLikeIdRoundTrips();
@@ -114,6 +152,10 @@ int main() {
     testBase64RoundTripAllBytes();
     testBase64DecodeRejectsMalformed();
 
+    testTcpMcpPanelIsHotspot();
+    testFxWindowStillHotspot();
+    testNonFxSurfacesAreNotHotspot();
+
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;
 }