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

src/core/ui/drag_out.cpp

@@ -50,8 +50,7 @@ PathList assemblePathList(const std::vector<ResolvedSample>& resolved) {
 
 OsHandoff decideOsHandoff(const std::vector<std::string>& paths) {
     OsHandoff out;
-    out.startOsDrag = !paths.empty();
-    out.releaseInternalDrag = out.startOsDrag;
+    out.handOffToOs = !paths.empty();
     return out;
 }
 

src/core/ui/drag_out.h

@@ -71,13 +71,15 @@ PathList assemblePathList(const std::vector<ResolvedSample>& resolved);
 
 // --- OS hand-off ordering -----------------------------------------------------
 
-// The two side effects the shell performs when a drag crosses out of REAPER. They are ONE
-// decision, not two: winding the internal drag down (release capture, clear drag state) for a
-// hand-off that then cannot happen consumes the gesture — the user sees a drag that silently
-// did nothing and drags again. Never release without starting.
+// Whether an empty/unresolvable payload should hand off to the OS at all. This decides ONLY
+// the empty-payload third of the failure space — an unresolvable payload must leave the
+// internal drag live rather than winding it down (release capture, clear drag state) for a
+// hand-off that then never happens, which reads to the user as "the drag did nothing, try
+// again". A resolved-but-OS-not-ready hand-off (OLE unavailable, HDROP build failure) is a
+// separate, shell-side readiness gate (drag_out_win::canInitiateDragOut) checked BEFORE the
+// shell tears down internal drag state — this struct does not model that path.
 struct OsHandoff {
-    bool startOsDrag = false;         // hand `paths` to the OS drag initiator
-    bool releaseInternalDrag = false; // first wind down mouse capture + panel drag state
+    bool handOffToOs = false;  // true: wind down internal drag state, then start the OS drag
 };
 
 // Decides the hand-off from the assembled path list. Empty (everything stale/unresolvable)