Hand a card dragged onto a plugin's own editor to the OS as a file drop; withdraw the OLE drag when the pointer returns to REAPER

A code-ownership proof (window class procedure's module, via the pure window_origin) plus REAPER's FX-surface token routes a plugin editor to OLE; FX chrome keeps the instrument drop. The OLE loop re-asks the law each turn, so passing a plugin editor stays reversible.

src/core/ui/window_origin.cpp

@@ -0,0 +1,48 @@
+// window_origin — pure implementation. See window_origin.h.
+
+#include "core/ui/window_origin.h"
+
+#include <algorithm>
+
+namespace reasampler::ui {
+
+namespace {
+
+// Case-folded (ASCII only — Windows' own directory names are ASCII), separator-unified, with the
+// "\\?\" long-path prefix GetModuleFileName may report dropped, so the same directory compares
+// equal however each API chose to spell it.
+std::string normalizePath(const std::string& p) {
+    std::string s = p.rfind("\\\\?\\", 0) == 0 ? p.substr(4) : p;
+    for (char& c : s) {
+        if (c == '/') c = '\\';
+        else if (c >= 'A' && c <= 'Z') c = static_cast<char>(c - 'A' + 'a');
+    }
+    while (!s.empty() && s.back() == '\\') s.pop_back();
+    return s;
+}
+
+// Directory containment, not string prefix: C:\Windows must not claim C:\WindowsApps.
+bool isBeneath(const std::string& path, const std::string& dir) {
+    const std::string p = normalizePath(path);
+    const std::string d = normalizePath(dir);
+    return !d.empty() && p.size() > d.size() && p.compare(0, d.size(), d) == 0 &&
+           p[d.size()] == '\\';
+}
+
+}  // namespace
+
+WindowCode classifyWindowCode(const WindowFacts& facts, const std::string& systemRoot) {
+    // Another process's procedure address means nothing in ours; the process boundary is itself
+    // the proof. Inside an FX window that is the bridged plugin's editor.
+    if (facts.otherProcess) return WindowCode::Plugin;
+    if (!facts.procResolved) return WindowCode::Unknown;
+    if (facts.hostModule) return WindowCode::Host;
+    if (normalizePath(systemRoot).empty()) return WindowCode::Unknown;
+    return isBeneath(facts.modulePath, systemRoot) ? WindowCode::System : WindowCode::Plugin;
+}
+
+bool isPluginEditorChain(const std::vector<WindowCode>& chain) {
+    return std::find(chain.begin(), chain.end(), WindowCode::Plugin) != chain.end();
+}
+
+}  // namespace reasampler::ui