Prove the render bounds at the boundary they cross, and name a short render whose count is exactly a millisecond-floored window

No truncation exists on our side of that boundary, so the read-back is the
only evidence available for whether REAPER kept the window — and it fires on
every tail mode, where only None was ever judged.

src/shell/capture/capture.cpp

@@ -34,6 +34,7 @@
 #include "core/capture/wav_codec.h"   // hashWavContent / collapseToMono — the one WAV/RIFF owner
 #include "core/util/file_bytes.h"
 #include "core/capture/render_settings.h"
+#include "core/capture/render_window.h"        // describeBoundsDrift — the read-back's verdict
 #include "shell/capture/render_bounds_gate.h"  // the exact-bounds verdict on the landed render
 
 #define REAPERAPI_MINIMAL
@@ -424,6 +425,12 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
     GetSetProjectInfo(proj, "RENDER_STARTPOS", request.startSeconds, true);
     GetSetProjectInfo(proj, "RENDER_ENDPOS", request.endSeconds, true);
 
+    // The requested window crosses out of this process HERE and nowhere else, so the
+    // read-back is the only evidence available on this side of that boundary for
+    // whether REAPER kept it. Reported below, once the project rate is known.
+    const double storedStart = GetSetProjectInfo(proj, "RENDER_STARTPOS", 0.0, false);
+    const double storedEnd   = GetSetProjectInfo(proj, "RENDER_ENDPOS", 0.0, false);
+
     // TAILFLAG/TAILMS/NORMALIZE/TRIMEND from the pure mapping: None -> exact
     // bounds + disable-all normalize; Auto -> 8s tail + surgical trim-end
     // normalize + -72 dB TRIMEND; Manual -> clamped fixed tail + disable-all, no
@@ -453,6 +460,18 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
         GetSetProjectInfo(proj, "RENDER_SRATE",
                           static_cast<double>(effectiveSampleRate), true);
     }
+
+    // Silent unless a bound came back changed. Fires on EVERY tail mode on purpose:
+    // only None is judged against its window after the render, so this is the sole
+    // signal an Auto/Manual capture was shortened before it ever started.
+    {
+        const std::string drift =
+            describeBoundsDrift(request.startSeconds, request.endSeconds,
+                                storedStart, storedEnd, effectiveSampleRate);
+        if (!drift.empty())
+            ShowConsoleMsg(("ReaSampler capture: " + drift + "\n").c_str());
+    }
+
     GetSetProjectInfo(proj, "RENDER_CHANNELS",
                       static_cast<double>(request.channelCount), true);
 

src/shell/capture/render_bounds_gate.cpp

@@ -99,6 +99,17 @@ BoundsVerdict checkRenderedBounds(const std::string& renderedPath,
     const long long actualFrames = static_cast<long long>(layout.frameCount());
     if (renderHonoredBounds(expectedFrames, actualFrames)) return v;
 
+    // Says whether this shortfall has the one shape two live short renders already
+    // matched to the frame, so every refusal from here on adds to (or breaks) that
+    // evidence instead of needing the arithmetic done by hand. A count coincidence
+    // only — it does not establish how the render resolved anything.
+    const std::string msNote =
+        actualFrames == msFlooredEndFrameCount(request.startSeconds,
+                                               request.endSeconds, rate)
+            ? " Those are exactly the frames this window holds with its end floored to"
+              " the millisecond -- a match on the count, not a measured cause."
+            : std::string();
+
     v.refused = true;
     v.message = "Render produced " + std::to_string(actualFrames) +
                 " frames but the requested range is " + std::to_string(expectedFrames) +
@@ -108,7 +119,7 @@ BoundsVerdict checkRenderedBounds(const std::string& renderedPath,
                 std::to_string(request.endSeconds) + "s) -> frame indices [" +
                 std::to_string(std::llround(request.startSeconds * rate)) + ", " +
                 std::to_string(std::llround(request.endSeconds * rate)) + ")." +
-                retainRefusedRender(renderedPath, projectDir);
+                msNote + retainRefusedRender(renderedPath, projectDir);
     return v;
 }