Close the bridged-process hole in the drag-out hand-off gate

OsHandoff now needs the window-ownership proof AND a hit-test that named
nothing, so a bridged plugin's UI can't read as off-REAPER.

src/shell/actions/CLAUDE.md

@@ -41,8 +41,8 @@ is owned by other directories and only skinned here.
 
 - `shell/actions` (`action_registry` / `design_view_actions` / `bank_actions` / `prune_action`) — the bindable action families, all routed via the `command_id`/`gaccel`/`hookcommand` contract. `action_registry` owns the shared registration plumbing (interned channel-qualified id strings; register and mirror-unregister present the identical pointer) **and the Q-W6 registration TABLE**: `main.cpp`'s own family (capture scopes, panel toggle, insert, batch, realtime, recapture, version) is one `ActionTableRow` array — suffix, phrase, flat function-pointer handler — that registration, hookcommand dispatch, and the unload mirror-unregister all iterate, so adding an action touches the table only (OCP). Bank mutations flow through the promptless `shell/bank_ops` verbs (`bankOp*` + `persistBankOp`, taking `ReaSamplerSession&`), which the panel menus and `bank_actions` consume as thin UX skins. **Every bank index verb wraps its mutation in a batched REAPER undo point (`Undo_BeginBlock2`/`EndBlock2`, `UNDO_STATE_MISCCFG`) so one bank operation is one Ctrl-Z.** The prune action (`prune_action`, `BANK_PRUNE_FOLDER`) is **the ONLY file-deletion action in the system**; it opens no undo point (file deletion is not REAPER-undoable). `BANK_PRUNE_FOLDER` halts on `blockedByTracking` and prints each blocker that fired, with recovery instructions.
 - `package_export_action` — the "export bank as package" skin: survey and report first, confirm what is absent (and, separately, a destination being replaced), pick a destination, write. Every prompt in the flow lives here so `shell/package/export_bank` stays promptless. Read-only against the project — it holds the session by `const&`, so no ext-state write, generation bump or undo point is reachable. Registration rides `main.cpp`'s action table (`EXPORT_BANK_PACKAGE`); the panel's tab menu is the second skin over the same body.
-- `drag_out_win` — the host/OS boundary of drag-out: Windows OLE `DoDragDrop`/`CF_HDROP`, copy-only (`DROPEFFECT_MOVE` not offered); macOS/Linux via `SWELL_InitiateDragDropOfFileList`. Also owns `pointerOverHostWindow`, the window-ownership test behind `ui::DropContext::pointerOffHost` — process identity on Windows (`WindowFromPoint` + `GetWindowThreadProcessId`), non-null `WindowFromPoint` under SWELL, whose window list is already this-process-only.
-- `instrument_drop_win` — instrument-drop shell: `probeDropTarget` resolves a screen point to a track + a `ReaperSurface` (via the pure `wire::classifyReaperSurface`, whose token rules `core/wire/CLAUDE.md` owns) — it answers only what REAPER named, never whether the pointer is still in REAPER, which is `drag_out_win`'s — and the drop half adds a ReaSampler 9000 instance and applies the dragged capture's state via a transient `.vstpreset` + `TrackFX_SetPreset` (the former `TrackFX_SetNamedConfigParm` "vst_chunk" write was silently unappliable for VST3). Exposes `loadInstrumentOntoTrack` (inner half, no own undo block) and `performInstrumentDrop` (wraps in its own undo block). **Never captures, never writes the bank, never inserts a timeline item.**
+- `drag_out_win` — the host/OS boundary of drag-out: Windows OLE `DoDragDrop`/`CF_HDROP`, copy-only (`DROPEFFECT_MOVE` not offered); macOS/Linux via `SWELL_InitiateDragDropOfFileList`. Also owns `pointerOverHostWindow`, the window-ownership test behind `ui::DropContext::pointerOffHost` (`core/ui/CLAUDE.md` owns the hand-off rule it feeds) — process identity on Windows (`WindowFromPoint` + `GetWindowThreadProcessId`), non-null `WindowFromPoint` under SWELL, whose window list is already this-process-only. Its shipped exception: a plugin bridged into `reaper_host*.exe` reports off-host though the pointer never left REAPER.
+- `instrument_drop_win` — instrument-drop shell: `probeDropTarget` resolves a screen point to a track + a `ReaperSurface` (via the pure `wire::classifyReaperSurface`, whose token rules `core/wire/CLAUDE.md` owns) — it answers only what REAPER named, never whether the pointer is still in REAPER — and the drop half adds a ReaSampler 9000 instance and applies the dragged capture's state via a transient `.vstpreset` + `TrackFX_SetPreset` (the former `TrackFX_SetNamedConfigParm` "vst_chunk" write was silently unappliable for VST3). Exposes `loadInstrumentOntoTrack` (inner half, no own undo block) and `performInstrumentDrop` (wraps in its own undo block). **Never captures, never writes the bank, never inserts a timeline item.**
 - `arrange_drop_win` — the drag-out gesture's arrange outcome: `arrangeTimeAtScreenX` (pointer column → time via `GetSet_ArrangeView2`'s one-pixel-span reading — inferred, not SDK-documented) and `performArrangeDrop` (snap the drop time, then one `InsertMedia` per capture on the pointer's track — assumed, not confirmed, to land end-to-end via REAPER's own cursor advance — in ONE undo block, counting only InsertMedia's reported successes, with the caller's track selection and edit cursor restored). The one timeline-placing shell here, per the invariant above; it never captures and never writes the bank.
 - `package_import_action` — the bindable / bank-menu / file-drop skin over `shell/package`'s `importBankPackage`. Owns the **ledger gate**, which runs BEFORE the file picker (a refusal must not cost the user a file choice) and is keyed on the session's `LedgerStatus` alone — never on `PruneReport::blockedByTracking`, whose undecodable-`rsusage_*` arm governs deletion-time protection and would refuse an import that only writes birth records. Builds and shows every message the import produces, but the ledger-refusal body itself is `core/package::ledgerRefusalMessage` — a pure fold this TU only supplies the channel-correct namespace to — so the wording is assertable without a DAW. `doImportBankPackage`/`doImportBankPackageFile` return the minted bank id on a landed import (empty otherwise) so a caller can focus it; the verb itself is promptless.
 - `ingest` — ingest-through-the-bank shell on the EXTENSION side: three surfaces — (1) arrange capture→bank→assign (bindable action), (2) Media-Explorer import→bank→instrument on the selected track, (3) file drop onto the bank panel→bank only. Only surface (1) writes the `assignment_request` ext-state wire. **ingest NEVER inserts a timeline item.** Surface (2)'s action is the one in this directory published into a NON-main action section (Media Explorer) as well as Main — two ids, one handler, two dispatch hooks; see root `CLAUDE.md` §"REAPER extension contract" for the mechanism.

src/shell/actions/drag_out_win.cpp

@@ -247,14 +247,18 @@ bool canInitiateDragOut(const std::vector<std::string>& absolutePaths) {
 bool pointerOverHostWindow(int screenX, int screenY) {
     // Process identity, not window identity: REAPER's arrange, its floating FX and VST editor
     // windows, its menus and its docker chrome are separate top-level HWNDs with no common
-    // ancestor, so no GetAncestor/GetParent walk reaches them all — but every one of them is
-    // created by the process this extension is loaded into.
+    // ancestor, so no GetAncestor/GetParent walk reaches them all — but they are windows of the
+    // process this extension is loaded into, bar the bridged-plugin case the header names.
     // Mouse capture does not affect WindowFromPoint: it hit-tests the desktop window tree, not
-    // input routing. It does skip hidden and DISABLED windows, so a REAPER window disabled
-    // behind a modal dialog could fall through to whatever sits beneath — no such dialog can be
-    // up while our panel holds capture for a drag.
+    // input routing. It does skip hidden and DISABLED windows, so a REAPER window disabled behind
+    // a modal dialog raised mid-drag (a REAPER timer or another extension can raise one — capture
+    // is not a modality lock) would fall through to whatever sits beneath and read as off-host.
+    // Accepted, unverified: not reproduced, and nothing here was checked against REAPER's own
+    // dialog behavior.
     const POINT pt{screenX, screenY};
     const HWND hwnd = WindowFromPoint(pt);
+    // Not an unknown resolved toward hand-off: a null return is the desktop tree stating there is
+    // no window at that point at all, which is a determinate "no host window here".
     if (!hwnd) return false;
     DWORD pid = 0;
     GetWindowThreadProcessId(hwnd, &pid);

src/shell/actions/drag_out_win.h

@@ -33,11 +33,14 @@ bool initiateDragOut(HWND__* panelHwnd, const std::vector<std::string>& absolute
 // probe HGLOBAL immediately; SWELL exposes no probe, so macOS/Linux reduces to the non-empty check.
 bool canInitiateDragOut(const std::vector<std::string>& absolutePaths);
 
-// True iff a window belonging to the host (REAPER) process sits under the screen point —
-// the POSITIVE proof ui::DropContext::pointerOffHost is the negation of. Every REAPER
-// surface qualifies whether or not GetThingFromPoint can name it: the arrange, a TCP, a
-// floating FX or VST editor window, the transport, the toolbar, a menu, the docker chrome
-// and our own panel are all windows of this process, because the extension runs inside it.
+// True iff a window belonging to the host (REAPER) process sits under the screen point — the
+// POSITIVE proof ui::DropContext::pointerOffHost is the negation of (that member owns the
+// evidence rule). Nearly every REAPER surface qualifies whether or not GetThingFromPoint can
+// name it, because the extension runs inside that process.
+// The exception, and it ships: a plugin set to "Run as separate/dedicated process" draws its
+// UI out of reaper_host32/64.exe, and WindowFromPoint descends to the deepest child — so this
+// reports false over a bridged plugin's editor although the pointer never left REAPER. The
+// law's hand-off gate absorbs that by also requiring REAPER's own hit-test to name nothing.
 // Side-effect-free; safe on every mouse-move.
 bool pointerOverHostWindow(int screenX, int screenY);
 

src/shell/actions/instrument_drop_win.h

@@ -35,8 +35,8 @@ struct DropProbe {
 // Wraps GetThingFromPoint: its track comes back as-is, its info string goes to the pure
 // wire::classifyReaperSurface. Cheap enough to run on every mouse-move, but the panel
 // evaluates it only OUTSIDE its own client rect — the internal drag never pays for it.
-// It reports only what REAPER named; whether the pointer is still inside REAPER at all is
-// drag_out_win::pointerOverHostWindow's, and no surface reported here implies an exit.
+// Reports only what REAPER named; no output here implies an exit (ui::DropContext::
+// pointerOffHost owns that rule).
 DropProbe probeDropTarget(int screenX, int screenY);
 
 // Adds a fresh ReaSampler 9000 instance to `track` and applies `presetBytes` as its