One undo-block opener for the whole tree: fourteen sites route through it, and the reclaim span can no longer swallow a user's act

The drain refuses a tick inside a foreign block; applyMode wakes the reclaimer
before it opens the span.

src/shell/actions/arrange_drop_win.cpp

@@ -29,6 +29,7 @@
 #include <vector>
 
 #include "core/capture/insert_plan.h"  // computeInsertMode — the ONE InsertMedia bitfield owner
+#include "shell/undo/undo_block.h"     // the ONE undo-block opener
 
 #include "reaper_plugin.h"
 
@@ -42,8 +43,6 @@
 #define REAPERAPI_WANT_SetOnlyTrackSelected
 #define REAPERAPI_WANT_SetTrackSelected
 #define REAPERAPI_WANT_SnapToGrid
-#define REAPERAPI_WANT_Undo_BeginBlock2
-#define REAPERAPI_WANT_Undo_EndBlock2
 #include "reaper_plugin_functions.h"
 
 namespace reasampler {
@@ -98,7 +97,7 @@ int performArrangeDrop(MediaTrack* track, double time,
 
     // One undo block around the whole drop (every item plus the selection/cursor restore) so a
     // single Ctrl-Z returns the project to exactly its pre-drop state.
-    Undo_BeginBlock2(nullptr);
+    UndoBlock undo(nullptr);
 
     SetOnlyTrackSelected(track);
     SetEditCurPos(SnapToGrid(nullptr, time), /*moveview=*/false, /*seekplay=*/false);
@@ -111,9 +110,6 @@ int performArrangeDrop(MediaTrack* track, double time,
         if (InsertMedia(path.c_str(), mode) != 0) ++inserted;
     }
 
-    // A fixed stack buffer, not std::string concatenation: the prior shape built the label with
-    // std::to_string + `+` between the restore below and Undo_EndBlock2, so a bad_alloc there
-    // would leave an unbalanced undo block open. snprintf here removes the allocation outright.
     char label[64];
     std::snprintf(label, sizeof(label), "ReaSampler: drop %d %s onto arrange", inserted,
                  inserted == 1 ? "capture" : "captures");
@@ -122,7 +118,7 @@ int performArrangeDrop(MediaTrack* track, double time,
     SetEditCurPos(priorCursor, /*moveview=*/false, /*seekplay=*/false);
 
     // extraflags -1 = UNDO_STATE_ALL, matching the insert action's own block.
-    Undo_EndBlock2(nullptr, label, -1);
+    undo.recordPoint(label, -1);
     return inserted;
 }