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/core/capture/CLAUDE.md

@@ -52,7 +52,7 @@ Detail specific to these pure modules:
 - `capture_name` — the REAPER-free composition of one capture's label + file-stem base from its source-track name(s), a local-calendar discriminator (`MM-DD HHMM`, from the shell's clock read), and an optional batch ordinal. The label and the stem deliberately diverge: the stem still passes through `capture_paths::sanitizeStem` (so a name that sanitizes to nothing files as `capture`), while the label keeps the source name verbatim. Stem uniqueness stays entirely `makeUniqueTag`'s — this module never disambiguates.
 - `insert_plan` — the REAPER-free logic behind the `insert` shell (M6): computes the `InsertMedia` `mode` bitmask from an `InsertOptions` struct (placement target, tempo-conform ratio, preserve-pitch flag), guaranteeing the &4 stretch-to-time-selection bit is never set and that no tempo bits are set when `conform == None`.
 - `render_settings` — the REAPER-free logic behind the capture action family: `SourceMode` → `RENDER_SETTINGS` bit mapping, `P_RAZOREDITS` string parsing + range-union bounds, razor-else-time range inference, the FX-scope bypass plan (`fxBypassPlanFor`), the tail-mode → `RENDER_TAILFLAG`/`RENDER_NORMALIZE`/`RENDER_TRIMEND` mapping (`tailRenderSettingsFor`) and its realtime-window analog (`realtimeRecordWindowEnd`), the capture-action taxonomy table (`captureActionTable`) `main.cpp` iterates to register the CAPTURE_ITEM/CAPTURE_TRACK family, and `renderSourceLabel` (the source named in the offline backend's bounds refusal).
-- `render_window` — the REAPER-free frame arithmetic behind exact capture bounds: `frameCountFor` (the frame count a project-time window occupies at the project rate — the number the offline backend checks the rendered file against before landing it, so a render that printed something other than the window is refused rather than banked), `renderHonoredBounds` (the gate's verdict and the sole home of its one-frame tolerance, which is empirical rather than proven — the header states which renderer models it covers and which it does not), and `itemExtentPrintsWindow`, the predicate `render_settings::sourceModeForScope` consults to decide whether REAPER's selected-items render source can express a requested window at all.
+- `render_window` — the REAPER-free frame arithmetic behind exact capture bounds: `frameCountFor` (the frame count a project-time window occupies at the project rate — the number the offline backend checks the rendered file against before landing it, so a render that printed something other than the window is refused rather than banked), `renderHonoredBounds` (the gate's verdict and the sole home of its one-frame tolerance, which is empirical rather than proven — the header states which renderer models it covers and which it does not), and `itemExtentPrintsWindow`, the predicate `render_settings::sourceModeForScope` consults to decide whether REAPER's selected-items render source can express a requested window at all. It also owns the two short-render diagnostics: `msFlooredEndFrameCount` (the frames a window holds with its end floored to the millisecond — the shape two live short renders matched, quoted by the refusal as a count coincidence and nothing more) and `describeBoundsDrift` (the sentence the offline backend prints when `RENDER_STARTPOS`/`RENDER_ENDPOS` do not read back as they were written).
 - `track_topology` — the REAPER-free folder arithmetic over a project's flat `I_FOLDERDEPTH` delta list: `directChildIndices` names a folder parent's DIRECT children, the set `shell/capture/render_isolation` silences so a ranged item capture does not print its track's children. Grandchildren are excluded by construction — they reach the parent only through the child that owns them.
 - `tail_control` — the REAPER-free logic behind the docked `bank_panel`'s tail-mode toggle: the cycle order (None → Auto → Manual → None), the Manual-length clamp/scroll-wheel fine-adjust (`clampManualMs`/`adjustManualMs`, 250 ms/notch, 2000 ms default), the toggle's label text (e.g. "Tail: Manual 2.0s"), and the `TailSetting` JSON round-trip persist stores per-project.
 

src/core/capture/render_window.cpp

@@ -3,6 +3,7 @@
 #include "core/capture/render_window.h"
 
 #include <cmath>
+#include <cstdio>
 
 namespace reasampler::capture {
 
@@ -15,6 +16,22 @@ long long frameIndexAt(double seconds, int sampleRate) {
     return std::llround(seconds * static_cast<double>(sampleRate));
 }
 
+// See the header for why whole milliseconds get a tolerance and why it is this small.
+double floorToMilliseconds(double seconds) {
+    const double ms      = seconds * 1000.0;
+    const double nearest = std::nearbyint(ms);
+    if (std::fabs(ms - nearest) < 1e-6) return nearest / 1000.0;
+    return std::floor(ms) / 1000.0;
+}
+
+// Full round-trip precision: a drift report whose two numbers print identically
+// would be evidence of nothing.
+std::string exactly(double seconds) {
+    char buf[32];
+    std::snprintf(buf, sizeof(buf), "%.17g", seconds);
+    return buf;
+}
+
 }  // namespace
 
 long long frameCountFor(double startSeconds, double endSeconds, int sampleRate) {
@@ -41,4 +58,29 @@ bool itemExtentPrintsWindow(double reqStart, double reqEnd,
         && frameIndexAt(reqEnd,   sampleRate) == frameIndexAt(itemEnd,   sampleRate);
 }
 
+long long msFlooredEndFrameCount(double startSeconds, double endSeconds,
+                                 int sampleRate) {
+    return frameCountFor(startSeconds, floorToMilliseconds(endSeconds), sampleRate);
+}
+
+std::string describeBoundsDrift(double reqStart, double reqEnd,
+                                double storedStart, double storedEnd,
+                                int sampleRate) {
+    // Bit equality, deliberately: the caller wrote these exact doubles and read them
+    // straight back, so anything but the same bits is a value REAPER changed.
+    if (storedStart == reqStart && storedEnd == reqEnd) return {};
+
+    std::string s = "REAPER did not keep the render bounds it was handed -- asked for [" +
+                    exactly(reqStart) + "s, " + exactly(reqEnd) + "s), read back [" +
+                    exactly(storedStart) + "s, " + exactly(storedEnd) + "s).";
+    if (sampleRate > 0) {
+        s += " The stored window is " +
+             std::to_string(frameCountFor(storedStart, storedEnd, sampleRate)) +
+             " frames against the " +
+             std::to_string(frameCountFor(reqStart, reqEnd, sampleRate)) +
+             " the request asks for, at " + std::to_string(sampleRate) + " Hz.";
+    }
+    return s;
+}
+
 }  // namespace reasampler::capture

src/core/capture/render_window.h

@@ -1,9 +1,12 @@
 #pragma once
 // render_window — pure frame arithmetic for a capture's requested window: the
-// frame count a project-time range occupies, and whether a render whose bounds
-// come from the selected items' own extent already prints that window.
+// frame count a project-time range occupies, whether a render whose bounds come
+// from the selected items' own extent already prints that window, and the two
+// diagnostics that say where a short render lost its frames.
 // NO REAPER types; unit-tested by tests/test_render_window.cpp.
 
+#include <string>
+
 namespace reasampler::capture {
 
 // Frames the [startSeconds, endSeconds) window occupies at `sampleRate`. Both
@@ -42,4 +45,28 @@ bool itemExtentPrintsWindow(double reqStart, double reqEnd,
                             double itemStart, double itemEnd,
                             int sampleRate);
 
+// --- Diagnostics: where a short render lost its frames ------------------------
+
+// The frames this window would hold if its END were resolved on a whole-millisecond
+// grid, floored, instead of exactly. Two live short renders (48 kHz, TailMode::None)
+// matched this count to the frame, which is the entire reason it exists.
+//
+// A COINCIDENCE OF COUNTS, not a claim about how anything resolved the end: nothing
+// renders from this number and no capture path asks for it. Whole-millisecond values
+// are recognized within a nanosecond, because a decimal millisecond is not always one
+// in binary (0.029 * 1000 lands just below 29) and a bare floor would drop a
+// millisecond from a window already on the grid. A nanosecond is far under one frame
+// at any rate we render, so a real sub-millisecond remainder still floors.
+long long msFlooredEndFrameCount(double startSeconds, double endSeconds,
+                                 int sampleRate);
+
+// The sentence a capture prints when the render bounds it handed REAPER did not read
+// back unchanged — the requested window, what came back, and both frame counts at
+// `sampleRate` (omitted when the rate is unknown). EMPTY when both edges read back
+// bit-identical, which is the only answer proving the request crossed into REAPER
+// intact; a caller prints this only when it is non-empty.
+std::string describeBoundsDrift(double reqStart, double reqEnd,
+                                double storedStart, double storedEnd,
+                                int sampleRate);
+
 }  // namespace reasampler::capture

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;
 }
 

tests/test_render_window.cpp

@@ -1,13 +1,14 @@
 // Standalone tests for reasampler::render_window — no REAPER, no framework.
 // Covers the bounds-equality number (a window's exact frame count at the project
-// rate), the verdict the offline backend refuses a capture on, and the predicate
+// rate), the verdict the offline backend refuses a capture on, the predicate
 // that decides whether REAPER's selected-items render source can express a
-// requested window at all.
+// requested window at all, and the two short-render diagnostics.
 
 #include "../src/core/capture/render_window.h"
 
 #include <cmath>
 #include <cstdio>
+#include <string>
 
 using namespace reasampler::capture;
 
@@ -15,6 +16,10 @@ static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
     std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
 
+static bool contains(const std::string& haystack, const std::string& needle) {
+    return haystack.find(needle) != std::string::npos;
+}
+
 // --- frameCountFor: the bounds equality, stated as a number ------------------
 
 static void testFrameCountIsExactNotRounded() {
@@ -226,6 +231,131 @@ static void testMultiItemUnionExtent() {
     CHECK(!itemExtentPrintsWindow(1.0, 4.0, 1.0, 9.0, 48000));
 }
 
+// --- msFlooredEndFrameCount: the shape both live short renders had ------------
+
+static void testMillisecondFlooredEndReproducesBothShortRenders() {
+    // Both DAW observations, as arithmetic. 48 kHz, TailMode::None, start at 0: the
+    // requested window's count, and the count its end floored to the millisecond
+    // holds — which is what each render actually printed.
+    CHECK(frameCountFor(0.0, 4.067797, 48000) == 195254);
+    CHECK(msFlooredEndFrameCount(0.0, 4.067797, 48000) == 195216);
+    CHECK(frameCountFor(0.0, 4.067797, 48000) -
+          msFlooredEndFrameCount(0.0, 4.067797, 48000) == 38);
+
+    CHECK(frameCountFor(0.0, 1.655172, 48000) == 79448);
+    CHECK(msFlooredEndFrameCount(0.0, 1.655172, 48000) == 79440);
+    CHECK(frameCountFor(0.0, 1.655172, 48000) -
+          msFlooredEndFrameCount(0.0, 1.655172, 48000) == 8);
+}
+
+static void testTheSixDecimalDisplayDidNotCreateTheEffect() {
+    // Both reported ends were printed to six decimals by the refusal. Each is one 4/4
+    // bar — at 59 BPM and at 145 BPM — so the full-precision doubles behind them are
+    // 240/59 and 240/145. Same counts either way: the display rounding is not what
+    // produces the shortfall.
+    CHECK(frameCountFor(0.0, 240.0 / 59.0, 48000) == 195254);
+    CHECK(msFlooredEndFrameCount(0.0, 240.0 / 59.0, 48000) == 195216);
+    CHECK(frameCountFor(0.0, 240.0 / 145.0, 48000) == 79448);
+    CHECK(msFlooredEndFrameCount(0.0, 240.0 / 145.0, 48000) == 79440);
+}
+
+static void testWindowAlreadyOnTheMillisecondGridLosesNothing() {
+    // The "sometimes it works" case: a bar at 120 BPM is exactly 2 s.
+    CHECK(msFlooredEndFrameCount(0.0, 2.0, 48000) == frameCountFor(0.0, 2.0, 48000));
+
+    // The binary-representation trap a bare floor would fall into. The premise, not an
+    // outcome: 1.007 s is a whole millisecond that really does land BELOW 1007 ms in
+    // double, so flooring it without a tolerance drops a millisecond from a window
+    // already on the grid.
+    CHECK(1.007 * 1000.0 < 1007.0);
+    CHECK(frameCountFor(0.0, 1.007, 48000) == 48336);
+    CHECK(msFlooredEndFrameCount(0.0, 1.007, 48000) == 48336);
+    // Same end reached from a non-zero start, so nothing here rests on the window
+    // beginning at 0.
+    CHECK(msFlooredEndFrameCount(0.5, 1.007, 48000) ==
+          frameCountFor(0.5, 1.007, 48000));
+}
+
+static void testOneFrameOfRemainderStillFloors() {
+    // The whole-millisecond tolerance must sit far below a frame, or it would swallow
+    // the very remainder this diagnostic exists to find. One frame at 48 kHz is 20.8 us
+    // — four orders of magnitude above the nanosecond tolerance.
+    const double oneFrame = 1.0 / 48000.0;
+    CHECK(frameCountFor(0.0, 1.0 + oneFrame, 48000) == 48001);
+    CHECK(msFlooredEndFrameCount(0.0, 1.0 + oneFrame, 48000) == 48000);
+}
+
+static void testMillisecondFloorAt44100WhereAMillisecondIsNotWholeFrames() {
+    // 44.1 kHz: a millisecond is 44.1 frames, so a floored end cannot be described as
+    // dropping a whole number of frames — the count still resolves exactly.
+    CHECK(frameCountFor(0.0, 0.0105, 44100) == 463);
+    CHECK(msFlooredEndFrameCount(0.0, 0.0105, 44100) == 441);
+    // And a window that IS on the millisecond grid there is untouched, even though its
+    // edge is not on a frame boundary.
+    CHECK(frameCountFor(0.0, 0.010, 44100) == 441);
+    CHECK(msFlooredEndFrameCount(0.0, 0.010, 44100) == 441);
+}
+
+static void testASubMillisecondStartWouldNotHideItself() {
+    // Both observations started at 0.000000s, the one value that hides a start-side
+    // truncation. A window whose START carries a sub-millisecond remainder counts from
+    // that exact start...
+    const double start = 1.0001724, end = 2.0001724;
+    CHECK(frameCountFor(start, end, 48000) == 48000);
+    // ...so a start floored to the millisecond would print a DIFFERENT count — 8 frames
+    // more, the same remainder the second observation lost off its end. A start-side
+    // truncation is therefore visible to the same frame-count gate, not silent.
+    CHECK(frameCountFor(1.000, end, 48000) == 48008);
+    CHECK(!renderHonoredBounds(frameCountFor(start, end, 48000),
+                               frameCountFor(1.000, end, 48000)));
+}
+
+// --- describeBoundsDrift: the read-back's verdict ------------------------------
+
+static void testBoundsThatReadBackUnchangedDescribeNothing() {
+    // The answer that proves the request crossed into REAPER intact — including for a
+    // window whose end is nowhere near a millisecond boundary.
+    CHECK(describeBoundsDrift(0.0, 4.067797, 0.0, 4.067797, 48000).empty());
+    CHECK(describeBoundsDrift(1.0001724, 2.0001724, 1.0001724, 2.0001724, 48000).empty());
+}
+
+static void testADriftedEndNamesBothWindowsAndBothCounts() {
+    const std::string s =
+        describeBoundsDrift(0.0, 4.067797, 0.0, 4.067, 48000);
+    CHECK(!s.empty());
+    // Both counts as literals from the DAW observation, not re-derived from the same
+    // functions the sentence was built with.
+    CHECK(contains(s, "195254"));   // what the request asks for
+    CHECK(contains(s, "195216"));   // what the drifted window would hold
+    CHECK(contains(s, "48000 Hz"));
+}
+
+static void testTheReportPrintsEnoughDigitsToShowTheDrift() {
+    // A report whose two numbers print identically is evidence of nothing. Two ends a
+    // single ULP apart — far under the sixth decimal a shorter rendering would stop at
+    // — must still read as two different numbers.
+    const double asked  = 4.067797;
+    const double stored = std::nextafter(asked, 5.0);
+    const std::string s = describeBoundsDrift(0.0, asked, 0.0, stored, 48000);
+    CHECK(!s.empty());
+    CHECK(!contains(s, "4.067797s, read back [0s, 4.067797s)"));
+}
+
+static void testADriftedStartIsCaughtToo() {
+    // The edge both observations could not test.
+    const std::string s = describeBoundsDrift(1.0001724, 2.0, 1.000, 2.0, 48000);
+    CHECK(!s.empty());
+    CHECK(contains(s, "1.0001724"));
+}
+
+static void testAnUnknownRateStillReportsTheDriftWithoutFrames() {
+    // A project that never pinned a rate reads 0. The drift is still worth saying; a
+    // frame count over an unknown rate is not.
+    const std::string s = describeBoundsDrift(0.0, 4.067797, 0.0, 4.067, 0);
+    CHECK(!s.empty());
+    CHECK(!contains(s, "frames"));
+}
+
 int main() {
     testFrameCountIsExactNotRounded();
     testFrameCountIsADifferenceOfIndicesNotADuration();
@@ -244,6 +374,17 @@ int main() {
     testSubFrameDriftStillPrintsTheSameFrames();
     testUnknownRateFallsBackToExactEquality();
     testMultiItemUnionExtent();
+    testMillisecondFlooredEndReproducesBothShortRenders();
+    testTheSixDecimalDisplayDidNotCreateTheEffect();
+    testWindowAlreadyOnTheMillisecondGridLosesNothing();
+    testOneFrameOfRemainderStillFloors();
+    testMillisecondFloorAt44100WhereAMillisecondIsNotWholeFrames();
+    testASubMillisecondStartWouldNotHideItself();
+    testBoundsThatReadBackUnchangedDescribeNothing();
+    testADriftedEndNamesBothWindowsAndBothCounts();
+    testTheReportPrintsEnoughDigitsToShowTheDrift();
+    testADriftedStartIsCaughtToo();
+    testAnUnknownRateStillReportsTheDriftWithoutFrames();
 
     if (g_fail) { std::printf("%d check(s) FAILED\n", g_fail); return 1; }
     std::printf("render_window: all checks passed\n");