S17 drop-and-load onto track FX button; S13 editor drop-accept (relay degraded)

S17: drag_out InstrumentDrop gesture + instrument_drop blob reusing the
instrument's own serializer; bank_panel FX hover-track + add-VST/vst_chunk
inject. S13 relay deferred (read-only bridge) — editor shows drop affordance.

tests/test_drag_out.cpp

@@ -84,6 +84,54 @@ static void testOffsetPanelRect() {
     CHECK(decideGesture(100, 19, p, s) == DragGesture::OsDrag);  // just above origin
 }
 
+// --- S17 InstrumentDrop gesture (single-capture over REAPER UI) ---------------
+//
+// M11 REGRESSION GUARD (load-bearing): every M11 case above uses DragState{true, true},
+// which leaves singleCapture=overReaperUi=false — so an M11-era payload outside the client
+// rect still decides OsDrag exactly as before. The tests above ARE the M11 non-regression
+// proof; these add the new middle case.
+
+// A SINGLE-capture drag that has left the panel but is still over REAPER's own UI is an
+// instrument drop (heading for a track's FX button), NOT an OS drag.
+static void testSingleCaptureOverReaperUiIsInstrumentDrop() {
+    DragState s{/*dragging=*/true, /*hasArmedSamples=*/true,
+                /*singleCapture=*/true, /*overReaperUi=*/true};
+    CHECK(decideGesture(500, 150, kPanel, s) == DragGesture::InstrumentDrop);  // right of panel
+    CHECK(decideGesture(-5, 150, kPanel, s) == DragGesture::InstrumentDrop);   // left of panel
+    CHECK(decideGesture(200, 400, kPanel, s) == DragGesture::InstrumentDrop);  // below
+}
+
+// InstrumentDrop is an OUTSIDE-only refinement: the same single-capture state INSIDE the
+// client rect is still the unchanged Internal bank-to-bank drag (invariant #4).
+static void testSingleCaptureInsidePanelStaysInternal() {
+    DragState s{true, true, /*singleCapture=*/true, /*overReaperUi=*/true};
+    CHECK(decideGesture(200, 150, kPanel, s) == DragGesture::Internal);
+}
+
+// A single-capture drag that has left REAPER ENTIRELY (overReaperUi=false) falls through to
+// OsDrag — the M11 OS drag-out to Explorer/another DAW, unchanged. This is the boundary
+// refinement's other half: leaving the client rect no longer immediately means OS-bound.
+static void testSingleCaptureOffReaperIsOsDrag() {
+    DragState s{true, true, /*singleCapture=*/true, /*overReaperUi=*/false};
+    CHECK(decideGesture(500, 150, kPanel, s) == DragGesture::OsDrag);
+}
+
+// A MULTI-capture drag over REAPER's UI is REJECTED for InstrumentDrop (the S17 open-question
+// lean): it is NOT a single instrument placement, so it falls through to OsDrag even while
+// over REAPER's UI — the multi-file drag-out is the natural gesture for a multi payload.
+static void testMultiCaptureOverReaperUiIsOsDrag() {
+    DragState s{true, true, /*singleCapture=*/false, /*overReaperUi=*/true};
+    CHECK(decideGesture(500, 150, kPanel, s) == DragGesture::OsDrag);
+}
+
+// Not-dragging / no-armed-samples still short-circuits to None regardless of the S17 fields.
+static void testS17FieldsIgnoredWhenNotDragging() {
+    CHECK(decideGesture(500, 150, kPanel,
+                        DragState{false, true, true, true}) == DragGesture::None);
+    CHECK(decideGesture(500, 150, kPanel,
+                        DragState{true, false, true, true}) == DragGesture::None);
+}
+
 // --- Path-list assembly -------------------------------------------------------
 
 static ResolvedSample ok(const std::string& p) { return ResolvedSample{p, true}; }
@@ -180,6 +228,12 @@ int main() {
     testReentryReturnsInternal();
     testOffsetPanelRect();
 
+    testSingleCaptureOverReaperUiIsInstrumentDrop();
+    testSingleCaptureInsidePanelStaysInternal();
+    testSingleCaptureOffReaperIsOsDrag();
+    testMultiCaptureOverReaperUiIsOsDrag();
+    testS17FieldsIgnoredWhenNotDragging();
+
     testSinglePath();
     testMultiPreservesOrder();
     testDedupeSamePath();