Defer Design View's per-FX park to an idle tick; honest undo mask, compare-before-write, PreventUIRefresh bracket, O(1) handle resolve

src/shell/view/CLAUDE.md

@@ -45,9 +45,16 @@ decide membership or mode rules.
   mode ids — no absolute paths, no index positions).
 - **Documented caveat:** offlined FX re-instantiate when a track returns to the
   active mode — stateful plugins (convolution, loaded samplers, tail-holding
-  effects) re-initialize on return (possible load hitch, un-persisted internal
-  state lost). Accepted cost of the CPU reclaim; surfaced at the toggle affordance
-  (tooltip).
+  effects) re-initialize on return (load hitch, un-persisted internal state lost).
+  Accepted cost of the CPU reclaim; surfaced at the toggle affordance (tooltip).
+  **The hitch no longer sits on the switch's synchronous path:** per-FX
+  offline/online is enqueued and applied on a later idle tick (`view_fx_park`),
+  so the new mode paints first. The deferral changes only WHEN the plugins move —
+  they still unload and re-instantiate, and un-persisted internal state is still
+  lost. What it does change is the undo record: the offline writes land outside
+  the switch's undo block, so the tool no longer re-drives them on an undo or a
+  redo — what a Ctrl-Z then leaves the chain at is REAPER's own FX-state record,
+  `[verify — DAW]`.
 - **Show-both semantics:** a per-track "pin visible across modes" flag re-enables
   processing whenever shown. A show-both leaf appears in every mode's visible set
   and is never parked — its driven flags stay at snapshot/restored values, FX
@@ -85,7 +92,8 @@ applies the resulting lane state to live tracks.
 
 ## Modules
 
-- `view` — Design View shell: snapshots flag values before parking, drives hide + CPU-park on inactive-mode leaves (`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN` + per-FX offline), restores from snapshot. Owns the one discriminator (`target != active`) that separates a real switch from a reapply, and with it both the playback gate (`transportBlocksModeSwitch`) and the solo cache/clear/restore seams. **Never touches master or `B_MUTE`.**
+- `view` — Design View shell: snapshots flag values before parking, drives hide + CPU-park on inactive-mode leaves (`B_SHOWINTCP`/`B_SHOWINMIXER`/`B_MAINSEND`/`I_FXEN`, with per-FX offline deferred to `view_fx_park`), restores from snapshot. Owns the one discriminator (`target != active`) that separates a real switch from a reapply, and with it both the playback gate (`transportBlocksModeSwitch`) and the solo cache/clear/restore seams. **Never touches master or `B_MUTE`.**
+- `view_fx_park` — the per-FX offline surface: the `TrackFX_GetFXGUID` identity read snapshot/park/restore share, the deferred intent queue that keeps `TrackFX_SetOffline` off the switch's synchronous path (at most one intent per track GUID, latest wins, an intent landing on its own pending inverse cancels it), and the idle-tick drain `main.cpp`'s `OnTimer` calls. The drain is also where a consumed snapshot is cleared — not where the restore was planned — so a second switch arriving before the first drained re-parks against the still-true captured state instead of re-capturing parked values.
 - `view_solo` — the `I_SOLO` read/write pair behind the per-mode solo surface, plus `clearTrackSolos`/`restoreTrackSolos`, the outgoing-clear and incoming-replay entry points `view` drives them through. Holds no policy: what to cache, clear, or replay is `core/view/solo_cache`.
 
 ## Gotchas
@@ -105,7 +113,7 @@ applies the resulting lane state to live tracks.
   the solo cache inherits it rather than introducing it. Not fixed here.
 - `fx_offline`'s identity keying (`TrackFX_GetFXGUID`) assumes the GUID stays
   attached to its plugin across a chain mutation while parked. That is
-  `[verify — DAW]` (see `fxGuidString` in `view.cpp`) and SWS issue #802 is a
+  `[verify — DAW]` (see `fxGuidString` in `view_fx_park.cpp`) and SWS issue #802 is a
   known reason it might not hold: `SNM_MoveOrRemoveTrackFX` reportedly leaves
   the FXID lines behind on reorder rather than moving them with the plugin. If
   confirmed, an SWS-driven reorder of a parked track's chain — not a native