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

src/shell/actions/drag_out_win.cpp

@@ -64,10 +64,11 @@ HGLOBAL buildHDrop(const std::vector<std::string>& paths) {
 
 // COM reference counts MUST be interlocked. A CF_HDROP target is free to marshal the data
 // object into another apartment and finish the copy on a background thread AFTER DoDragDrop
-// has returned (Explorer's async file copy does exactly this). A plain ++/-- there races the
-// source thread's post-DoDragDrop Release: one lost increment destroys the object — and with
-// it the source HGLOBAL — before the target reads it, and the drop lands with no file. That
-// race is intermittent and a retry usually wins it; do not "simplify" these back.
+// has returned; Explorer's async file copy is suspected to do exactly this, though that
+// specific behavior is not confirmed by experiment. A plain ++/-- there races the source
+// thread's post-DoDragDrop Release: one lost increment destroys the object — and with it the
+// source HGLOBAL — before the target reads it, and the drop lands with no file. That race is
+// intermittent and a retry usually wins it; do not "simplify" these back.
 inline ULONG comAddRef(volatile LONG& refs) {
     return static_cast<ULONG>(InterlockedIncrement(&refs));
 }
@@ -234,6 +235,15 @@ bool initiateDragOut(HWND__* /*panelHwnd*/, const std::vector<std::string>& abso
     return hr == DRAGDROP_S_DROP && effect == DROPEFFECT_COPY;
 }
 
+bool canInitiateDragOut(const std::vector<std::string>& absolutePaths) {
+    if (absolutePaths.empty()) return false;
+    if (!ensureOleForThisThread()) return false;
+    HGLOBAL hdrop = buildHDrop(absolutePaths);
+    if (!hdrop) return false;
+    GlobalFree(hdrop);  // probe only — initiateDragOut builds its own on the real attempt
+    return true;
+}
+
 }  // namespace reasampler
 
 #else  // ---- macOS / Linux (SWELL) -----------------------------------------------------
@@ -260,6 +270,13 @@ bool initiateDragOut(HWND__* panelHwnd, const std::vector<std::string>& absolute
     return true;  // fire-and-forget; SWELL owns the drag from here (no accept/cancel return)
 }
 
+// SWELL exposes no readiness probe ahead of SWELL_InitiateDragDropOfFileList (which itself
+// reports no accept/cancel outcome) — the non-empty check is the only thing knowable in
+// advance on this platform.
+bool canInitiateDragOut(const std::vector<std::string>& absolutePaths) {
+    return !absolutePaths.empty();
+}
+
 }  // namespace reasampler
 
 #endif