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/undo/undo_block.cpp

@@ -0,0 +1,51 @@
+// See undo_block.h. Compiled into the reaper_reasampler module; includes
+// reaper_plugin_functions.h without REAPERAPI_IMPLEMENT (main.cpp owns that).
+
+#include "shell/undo/undo_block.h"
+
+#include <utility>
+
+#include "shell/view/fx_reclaim.h"  // fxReclaimEndUndoSpan — the span this must not nest over
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_Undo_BeginBlock2
+#define REAPERAPI_WANT_Undo_EndBlock2
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+
+// Extension-owned nesting depth only — REAPER's own counter is not readable, and the
+// reclaim span is deliberately NOT counted here (it is the thing this stands off).
+int g_depth = 0;
+
+}  // namespace
+
+bool undoBlockOpen() { return g_depth > 0; }
+
+UndoBlock::UndoBlock(ReaProject* proj) : project_(proj) {
+    // ONLY the outermost block ends the span, which mechanizes the whole nesting rule.
+    // At depth 0 the span is the only block that can be open, so ending it mints the
+    // switch's point whole, at depth 0, before this block exists. At depth > 0 the
+    // enclosing UndoBlock already did it, and a close from here would land at depth
+    // 2->1 — minting nothing and hijacking the enclosing block's label instead. That is
+    // the same criterion applyMode's reapply is gated on.
+    if (g_depth++ == 0) fxReclaimEndUndoSpan();
+    Undo_BeginBlock2(proj);
+}
+
+UndoBlock::~UndoBlock() {
+    // Depth drops AFTER the SDK call, not before: the close that reaches depth 0 mints a
+    // point, and anything re-entered through that must still read a block as open rather
+    // than let the drain open a span underneath it.
+    Undo_EndBlock2(project_, label_.c_str(), mask_);
+    --g_depth;
+}
+
+void UndoBlock::recordPoint(std::string label, int mask) {
+    label_ = std::move(label);
+    mask_  = mask;
+}
+
+}  // namespace reasampler