diff --git a/.forgejo/workflows/tests.yml b/.forgejo/workflows/tests.yml
new file mode 100644
index 0000000..2f39cd9
--- /dev/null
+++ b/.forgejo/workflows/tests.yml
@@ -0,0 +1,28 @@
+name: tests
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  pytest:
+    runs-on: docker
+    container:
+      image: debian:bookworm
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: |
+          apt-get update
+          # PyGObject + GTK introspection are needed because the modules under
+          # test import gi.repository at import time; the tests themselves run
+          # headless (no display, no encoding).
+          apt-get install -y --no-install-recommends \
+            python3 python3-pip python3-gi gir1.2-gtk-3.0 \
+            python3-cairo python3-gi-cairo
+          pip3 install --break-system-packages pytest
+
+      - name: Run test suite
+        run: python3 -m pytest
diff --git a/.gitignore b/.gitignore
index ab7d506..683cc51 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@ __pycache__/
 build/
 dist/
 *.egg-info/
+.pytest_cache/
 
 # Compiled translations (generated from po/ at build time)
 src/devedeng/data/locale/
diff --git a/CLAUDE.md b/CLAUDE.md
index f9a58be..1ea5c02 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -26,21 +26,17 @@ Runtime external tools (must be on PATH): `ffmpeg`/`ffprobe` (or `avconv`/`avpro
 
 ## Testing
 
-There is **no test framework or test suite.** Verify changes two ways:
+There is a **pytest suite in `tests/`** (run `python3 -m pytest`; it needs `pytest` and PyGObject/`gi`). Tests are headless — no display, no actual encoding — and import the in-tree package via `tests/conftest.py` (which prepends `src/` to the path and installs the gettext `_` builtin). CI runs them on Codeberg via `.forgejo/workflows/tests.yml`. Coverage focuses on the two things that are cheap to test and easy to break:
 
-1. **Pure logic** (no GTK/display) — e.g. the split planner. Import and exercise directly:
-   ```bash
-   cd src && python3 -c "from devedeng.project import plan_disc_split; ..."
-   ```
-   `plan_disc_split`, `_movie_chapter_seconds`, and `_nearest_chapter_before` in `project.py` are deliberately module-level pure functions so they can be unit-tested without instantiating GTK.
+1. **Pure logic** (no GTK/display) — the split planner and bitrate math: `plan_disc_split`, `_movie_chapter_seconds`, `_nearest_chapter_before`, and `compute_high_profile_bitrate` in `project.py` are deliberately module-level pure functions so they can be unit-tested without instantiating GTK. See `tests/test_split_planner.py`, `tests/test_high_profile_bitrate.py`.
 
-2. **Generated ffmpeg commands** — build a converter and inspect `command_var` without running anything. The config singleton works headless:
+2. **Generated commands** — build a converter and assert on `command_var` without running anything (`tests/test_ffmpeg_command.py`); detection is checked by feeding `ffprobe.process_json` canned JSON (`tests/test_ffprobe_detection.py`). The `FakeFileProject`/`FakeMovie` factories and the `ffmpeg_converter`/`arg_after` helpers live in `conftest.py`. The config singleton works headless:
    ```python
    cfg = devedeng.configuration_data.configuration.get_config(); cfg.disc_type = "dvd"
    conv = devedeng.ffmpeg.ffmpeg(); conv.convert_file(mock_file_movie, "/tmp/out.mpg", 0)
    print(conv.command_var)   # the exact ffmpeg argv
    ```
-   `convert_file` takes the `file_movie` object as `file_project` and reads `hardsub_index`/`hardsub_kind`, the encode profile from config, etc. For a full integration check, run a real short encode through the executor's GLib loop and watch a fake progress bar (GTK is installed but there is no Xvfb here, so the *windowed* UI can only be verified by actually launching the app on a display).
+   `convert_file` reads `hardsub_index`/`hardsub_kind` off the `file_project` and the encode profile from config. For a full integration check, run a real short encode through the executor's GLib loop and watch a fake progress bar; the *windowed* UI can only be verified by launching the app on a real display.
 
 `contrib/dvd-safe-converter.sh` is the known-good baseline the **Compatibility** profile is modelled on — use it as a regression reference for the no-subtitle DVD encode (mpeg2video / 720x480 / 29.97 / AC3 192k / 48k).
 
diff --git a/pytest.ini b/pytest.ini
new file mode 100644
index 0000000..d8f1faf
--- /dev/null
+++ b/pytest.ini
@@ -0,0 +1,7 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+addopts = -q
+filterwarnings =
+    ignore::DeprecationWarning
+    ignore::PendingDeprecationWarning
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..ffa4df4
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,119 @@
+"""Shared test fixtures/helpers for the devedeng-extended test suite.
+
+These tests exercise pure logic and command generation without launching the
+GTK UI. They import the package from ``src/`` and stub out the gettext ``_``
+builtin so modules that call ``_(...)`` at runtime work headless.
+"""
+
+import os
+import sys
+import gettext
+
+# make the in-tree package importable without installing
+_SRC = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "src")
+if _SRC not in sys.path:
+    sys.path.insert(0, _SRC)
+
+# ensure the gettext _() builtin exists (the app normally installs it at start)
+gettext.install("devedeng")
+
+import pytest
+
+
+class FakeMovie:
+    """Minimal stand-in for file_movie for the pure split/bitrate planners.
+
+    Only the attributes those functions read are provided.
+    """
+
+    element_type = "file_movie"
+
+    def __init__(self, rate_kBps=586.5, length=3600, chapters_step=5,
+                 divide=True, manual=None, audio_streams=1, subtitles=0):
+        self._rate = rate_kBps
+        self.original_length = length
+        self.chapter_size = chapters_step
+        self.divide_in_chapters = divide
+        self.chapter_list_entry = manual
+        self.audio_streams = audio_streams
+        self.subtitles_list = [None] * subtitles
+
+    def get_kbytes_per_second(self):
+        return self._rate
+
+
+@pytest.fixture
+def make_movie():
+    """Factory for FakeMovie instances used by the planner tests."""
+    def _make(**kwargs):
+        return FakeMovie(**kwargs)
+    return _make
+
+
+class FakeFileProject:
+    """Stand-in for a file_movie passed to ffmpeg/avconv.convert_file.
+
+    Defaults describe a typical 1080p 16:9 source targeting NTSC DVD with no
+    hardsub. Override fields per test.
+    """
+
+    def __init__(self, **overrides):
+        self.file_name = "/movies/input.mkv"
+        self.title_name = "input"
+        self.two_pass_encoding = False
+        self.volume = 100
+        self.audio_delay = 0.0
+        self.copy_sound = False
+        self.no_reencode_audio_video = False
+        self.is_mpeg_ps = False
+        self.video_list = [0]
+        self.audio_list = [1]
+        self.deinterlace = "deinterlace_none"
+        self.prerotation = 0
+        self.rotation = "rotation_0"
+        self.mirror_vertical = False
+        self.mirror_horizontal = False
+        self.original_width = 1920
+        self.original_height = 1080
+        self.width_midle = 1920
+        self.height_midle = 1080
+        self.width_final = 720
+        self.height_final = 480
+        self.original_fps = 24
+        self.format_pal = False
+        self.sound5_1 = False
+        self.gop12 = True
+        self.video_rate_final = 4500
+        self.audio_rate_final = 192
+        self.aspect_ratio_final = 1.7777777777777777
+        self.original_length = 7200
+        self.hardsub_index = -1
+        self.hardsub_kind = "text"
+        self.embedded_subtitles = []
+        for k, v in overrides.items():
+            setattr(self, k, v)
+
+
+@pytest.fixture
+def make_file_project():
+    def _make(**overrides):
+        return FakeFileProject(**overrides)
+    return _make
+
+
+def ffmpeg_converter(disc_type="dvd", profile="compatibility"):
+    """Build an ffmpeg converter with the config singleton set for a test."""
+    import devedeng.configuration_data
+    import devedeng.ffmpeg
+    cfg = devedeng.configuration_data.configuration.get_config()
+    cfg.disc_type = disc_type
+    cfg.encode_profile = profile
+    return devedeng.ffmpeg.ffmpeg()
+
+
+def arg_after(cmd, flag):
+    """Return the argument following ``flag`` in an argv list, or None."""
+    for i, a in enumerate(cmd):
+        if a == flag and i + 1 < len(cmd):
+            return cmd[i + 1]
+    return None
diff --git a/tests/test_ffmpeg_command.py b/tests/test_ffmpeg_command.py
new file mode 100644
index 0000000..9f53a8e
--- /dev/null
+++ b/tests/test_ffmpeg_command.py
@@ -0,0 +1,136 @@
+"""Tests for ffmpeg.convert_file command generation (no encoding runs)."""
+
+from conftest import ffmpeg_converter, arg_after
+
+
+def _vf(cmd):
+    return arg_after(cmd, "-vf")
+
+
+def test_compatibility_dvd_basics(make_file_project):
+    conv = ffmpeg_converter(profile="compatibility")
+    conv.convert_file(make_file_project(), "/tmp/out.mpg", 0)
+    cmd = conv.command_var
+    assert "mpeg2video" in cmd
+    assert "ac3" in cmd                      # fork uses AC3 for NTSC DVD
+    assert arg_after(cmd, "-ar") == "48000"
+    assert arg_after(cmd, "-b:v") == str(4500 * 1000)
+    assert arg_after(cmd, "-b:a") == str(192 * 1000)
+    assert _vf(cmd) == "scale=720:480"
+    assert "-sn" in cmd                      # no soft-sub passthrough
+
+
+def test_runs_under_nice(make_file_project):
+    conv = ffmpeg_converter()
+    conv.convert_file(make_file_project(), "/tmp/out.mpg", 0)
+    assert conv.command_var[:4] == ["nice", "-n", "10", "ffmpeg"]
+
+
+def test_progress_goes_to_file_not_pipe(make_file_project):
+    # the deadlock fix: -progress must target a file, never pipe:N
+    conv = ffmpeg_converter()
+    conv.convert_file(make_file_project(), "/tmp/out.mpg", 0)
+    prog = arg_after(conv.command_var, "-progress")
+    assert prog is not None
+    assert not prog.startswith("pipe:")
+    assert prog.endswith(".progress")
+
+
+def test_text_hardsub_filter_after_scale(make_file_project):
+    fp = make_file_project(
+        hardsub_index=2, hardsub_kind="text",
+        embedded_subtitles=[{"index": 2, "codec": "subrip", "language": "eng",
+                             "title": "", "default": True, "forced": False,
+                             "kind": "text"}],
+    )
+    conv = ffmpeg_converter()
+    conv.convert_file(fp, "/tmp/out.mpg", 0)
+    vf = _vf(conv.command_var)
+    assert vf is not None
+    assert vf.startswith("scale=720:480")
+    assert "subtitles=" in vf
+    assert "si=0" in vf                       # first (and only) subtitle stream
+
+
+def test_text_hardsub_escapes_path(make_file_project):
+    fp = make_file_project(
+        file_name="/movies/My Movie: 2024.mkv",
+        hardsub_index=2, hardsub_kind="text",
+        embedded_subtitles=[{"index": 2, "codec": "subrip", "language": "eng",
+                             "title": "", "default": False, "forced": False,
+                             "kind": "text"}],
+    )
+    conv = ffmpeg_converter()
+    conv.convert_file(fp, "/tmp/out.mpg", 0)
+    vf = _vf(conv.command_var)
+    assert r"\:" in vf                        # the ':' in the path is escaped
+
+
+def test_image_hardsub_uses_filter_complex_overlay(make_file_project):
+    fp = make_file_project(
+        hardsub_index=3, hardsub_kind="image",
+        embedded_subtitles=[{"index": 3, "codec": "hdmv_pgs_subtitle",
+                             "language": "eng", "title": "", "default": True,
+                             "forced": False, "kind": "image"}],
+    )
+    conv = ffmpeg_converter()
+    conv.convert_file(fp, "/tmp/out.mpg", 0)
+    cmd = conv.command_var
+    fc = arg_after(cmd, "-filter_complex")
+    assert fc is not None
+    assert "overlay" in fc
+    # overlay must come before scale so bitmap subs stay aligned to the source
+    assert fc.index("overlay") < fc.index("scale=720:480")
+    assert fc.endswith("[vsub]")
+    assert arg_after(cmd, "-map") == "[vsub]"   # video taken from the graph
+    assert "-vf" not in cmd                      # not both -vf and filter_complex
+
+
+def test_high_profile_two_pass_and_448k_audio(make_file_project):
+    # regression for the AC3 448k -> 384k clamp bug
+    fp = make_file_project(two_pass_encoding=True,
+                           video_rate_final=3752, audio_rate_final=448)
+    conv = ffmpeg_converter(profile="high")
+    conv.convert_file(fp, "/tmp/out.mpg", 0)
+    cmd = conv.command_var
+    assert "-pass" in cmd and "-passlogfile" in cmd
+    assert arg_after(cmd, "-b:a") == str(448 * 1000)   # NOT clamped to 384k
+    assert arg_after(cmd, "-b:v") == str(3752 * 1000)
+    assert len(conv.childs) == 1                        # spawns the pass-2 child
+
+
+def test_audio_bitrate_table_allows_ac3_rates(make_file_project):
+    conv = ffmpeg_converter()
+    assert conv._adjust_audio_bitrate(448) == 448
+    assert conv._adjust_audio_bitrate(400) == 448      # rounds up to next valid
+    assert conv._adjust_audio_bitrate(192) == 192
+    assert conv._adjust_audio_bitrate(9999) == 640     # clamps to table max
+
+
+def test_even_dimensions_and_positive_crop_offset(make_file_project):
+    # odd letterbox height (1079) must be rounded even, offset positive
+    fp = make_file_project(width_midle=1920, height_midle=1079)
+    conv = ffmpeg_converter()
+    conv.convert_file(fp, "/tmp/out.mpg", 0)
+    cmd = conv.command_var
+    blob = _vf(cmd) or arg_after(cmd, "-filter_complex") or ""
+    assert "1079" not in blob                  # no odd dimension
+    assert ":-" not in blob                    # no negative crop/pad offset
+    assert "1078" in blob                      # rounded down to even
+
+
+def test_start_offset_emits_ss_before_inputs(make_file_project):
+    conv = ffmpeg_converter()
+    conv.convert_file(make_file_project(), "/tmp/out.mpg", 0, start_offset=120)
+    cmd = conv.command_var
+    # -ss appears before each -i (two inputs: audio-delay copy + video)
+    assert cmd.count("-ss") == 2
+    first_i = cmd.index("-i")
+    first_ss = cmd.index("-ss")
+    assert first_ss < first_i
+
+
+def test_no_start_offset_has_no_ss(make_file_project):
+    conv = ffmpeg_converter()
+    conv.convert_file(make_file_project(), "/tmp/out.mpg", 0)
+    assert "-ss" not in conv.command_var
diff --git a/tests/test_ffprobe_detection.py b/tests/test_ffprobe_detection.py
new file mode 100644
index 0000000..fc2adac
--- /dev/null
+++ b/tests/test_ffprobe_detection.py
@@ -0,0 +1,82 @@
+"""Tests for embedded subtitle detection in ffprobe.process_json."""
+
+import json
+import devedeng.ffprobe
+
+
+def _probe(streams):
+    p = devedeng.ffprobe.ffprobe()
+    data = json.dumps({"streams": streams})
+    err = p.process_json("/movies/input.mkv", data)
+    return p, err
+
+
+def _video(index=0, duration="120.0"):
+    return {"index": index, "codec_type": "video", "codec_name": "h264",
+            "width": 1920, "height": 1080, "avg_frame_rate": "24/1",
+            "duration": duration}
+
+
+def _audio(index=1):
+    return {"index": index, "codec_type": "audio", "codec_name": "aac",
+            "sample_rate": "48000"}
+
+
+def _sub(index, codec, language="eng", title="", default=0, forced=0):
+    return {"index": index, "codec_type": "subtitle", "codec_name": codec,
+            "tags": {"language": language, "title": title},
+            "disposition": {"default": default, "forced": forced}}
+
+
+def test_no_subtitles_empty_list():
+    p, err = _probe([_video(), _audio()])
+    assert err is False
+    assert p.subtitle_tracks == []
+
+
+def test_text_subtitle_classified_text():
+    p, _ = _probe([_video(), _audio(), _sub(2, "subrip", "eng", default=1)])
+    assert len(p.subtitle_tracks) == 1
+    t = p.subtitle_tracks[0]
+    assert t["index"] == 2
+    assert t["codec"] == "subrip"
+    assert t["language"] == "eng"
+    assert t["kind"] == "text"
+    assert t["default"] is True
+    assert t["forced"] is False
+
+
+def test_image_subtitles_classified_image():
+    for codec in ("hdmv_pgs_subtitle", "dvd_subtitle", "dvdsub", "pgssub", "xsub"):
+        p, _ = _probe([_video(), _audio(), _sub(2, codec)])
+        assert p.subtitle_tracks[0]["kind"] == "image", codec
+
+
+def test_various_text_codecs_classified_text():
+    for codec in ("subrip", "ass", "ssa", "mov_text", "webvtt"):
+        p, _ = _probe([_video(), _audio(), _sub(2, codec)])
+        assert p.subtitle_tracks[0]["kind"] == "text", codec
+
+
+def test_multiple_subtitle_tracks_in_order():
+    p, _ = _probe([
+        _video(), _audio(),
+        _sub(2, "subrip", "eng", default=1),
+        _sub(3, "hdmv_pgs_subtitle", "fre", forced=1),
+        _sub(4, "ass", "jpn", title="Signs"),
+    ])
+    assert [t["index"] for t in p.subtitle_tracks] == [2, 3, 4]
+    assert [t["kind"] for t in p.subtitle_tracks] == ["text", "image", "text"]
+    assert p.subtitle_tracks[1]["forced"] is True
+    assert p.subtitle_tracks[2]["title"] == "Signs"
+
+
+def test_missing_tags_and_disposition_defaults():
+    # a subtitle stream with no tags/disposition must not crash
+    p, _ = _probe([_video(), _audio(),
+                   {"index": 2, "codec_type": "subtitle",
+                    "codec_name": "subrip"}])
+    t = p.subtitle_tracks[0]
+    assert t["language"] == ""
+    assert t["default"] is False
+    assert t["forced"] is False
diff --git a/tests/test_high_profile_bitrate.py b/tests/test_high_profile_bitrate.py
new file mode 100644
index 0000000..b207e17
--- /dev/null
+++ b/tests/test_high_profile_bitrate.py
@@ -0,0 +1,67 @@
+"""Tests for the High-quality profile fill-the-disc bitrate math."""
+
+from devedeng.project import compute_high_profile_bitrate as calc
+
+CAP_KB = 3_780_000.0  # 4.2 GB * 0.90 margin, in kBytes
+FLOOR = 3000
+CAP = 9000
+
+
+def audio_kbits(rate_kbps, runtime_s, streams=1):
+    return rate_kbps * streams * runtime_s
+
+
+def test_short_movie_one_disc_at_cap():
+    # 30 min: tiny, so bitrate is pinned at the 9000k cap, one disc
+    n, br = calc(1800, audio_kbits(448, 1800), CAP_KB)
+    assert n == 1
+    assert br == CAP
+
+
+def test_two_hours_fills_one_disc():
+    n, br = calc(7200, audio_kbits(448, 7200), CAP_KB)
+    assert n == 1
+    assert FLOOR <= br <= CAP
+    # should be comfortably above the floor (filling the disc)
+    assert br > 3500
+
+
+def test_four_hours_two_discs():
+    n, br = calc(14400, audio_kbits(448, 14400), CAP_KB)
+    assert n == 2
+    assert FLOOR <= br <= CAP
+
+
+def test_eight_hours_multiple_discs():
+    n, br = calc(28800, audio_kbits(448, 28800), CAP_KB)
+    assert n >= 3
+    assert br >= FLOOR
+
+
+def test_zero_runtime_guard():
+    assert calc(0, 0, CAP_KB) == (1, CAP)
+
+
+def test_bitrate_always_within_bounds():
+    for rt in (300, 1800, 3600, 7200, 14400, 30000, 60000):
+        n, br = calc(rt, audio_kbits(448, rt), CAP_KB)
+        assert FLOOR <= br <= CAP, (rt, br)
+        assert n >= 1
+
+
+def test_filled_content_does_not_exceed_disc_capacity():
+    # the chosen bitrate must not overflow the n discs it claims to fill
+    for rt in (1800, 7200, 14400, 28800):
+        ak = audio_kbits(448, rt)
+        n, br = calc(rt, ak, CAP_KB)
+        used_kbits = br * rt + ak
+        assert used_kbits <= n * CAP_KB * 8 + 1  # +1 for rounding slack
+
+
+def test_menu_reservation_reduces_video_budget():
+    rt = 7200
+    ak = audio_kbits(448, rt)
+    n0, br0 = calc(rt, ak, CAP_KB, menu_kbytes=0)
+    n1, br1 = calc(rt, ak, CAP_KB, menu_kbytes=200_000)
+    # with a menu eating space, the fill bitrate can only be <= the no-menu one
+    assert br1 <= br0
diff --git a/tests/test_split_planner.py b/tests/test_split_planner.py
new file mode 100644
index 0000000..ee81e28
--- /dev/null
+++ b/tests/test_split_planner.py
@@ -0,0 +1,115 @@
+"""Tests for the pure disc-split planner in project.py."""
+
+from devedeng.project import (
+    plan_disc_split,
+    _movie_chapter_seconds,
+    _nearest_chapter_before,
+)
+
+# ~4.2 GB usable disc in kBytes (matches get_dvd_size()[0]*1000 with margin)
+CAP = 4_200_000.0
+# 4500k video + 192k audio = 4692 kbit/s -> /8 = 586.5 kByte/s
+RATE = (4500 + 192) / 8.0
+# seconds that fill one disc at RATE
+SECS_PER_DISC = CAP / RATE
+
+
+def names(plan):
+    return [[seg[0] for seg in disc] for disc in plan]
+
+
+def test_single_short_movie_one_disc(make_movie):
+    m = make_movie(rate_kBps=RATE, length=3600)
+    plan = plan_disc_split([m], CAP)
+    assert len(plan) == 1
+    assert plan[0] == [(m, 0, 0)]
+
+
+def test_two_movies_fit_one_disc(make_movie):
+    a = make_movie(rate_kBps=RATE, length=3000)
+    b = make_movie(rate_kBps=RATE, length=3000)
+    plan = plan_disc_split([a, b], CAP)
+    assert len(plan) == 1
+    assert names(plan) == [[a, b]]
+
+
+def test_two_movies_overflow_to_two_discs(make_movie):
+    a = make_movie(rate_kBps=RATE, length=5000)
+    b = make_movie(rate_kBps=RATE, length=5000)
+    plan = plan_disc_split([a, b], CAP)
+    assert len(plan) == 2
+    assert plan[0] == [(a, 0, 0)]
+    assert plan[1] == [(b, 0, 0)]
+
+
+def test_single_long_movie_splits_at_chapter(make_movie):
+    # 200 min movie, chapters every 5 min (300 s)
+    m = make_movie(rate_kBps=RATE, length=12000, chapters_step=5)
+    plan = plan_disc_split([m], CAP)
+    assert len(plan) >= 2
+    first = plan[0][0]
+    assert first[0] is m and first[1] == 0
+    cut = first[2]
+    # cut lands on a 300-s chapter boundary, just under one disc's capacity
+    assert cut % 300 == 0
+    assert SECS_PER_DISC - 300 < cut <= SECS_PER_DISC
+    # last segment runs to the end (duration 0)
+    assert plan[-1][0][2] == 0
+
+
+def test_single_long_movie_no_chapters_even_cut(make_movie):
+    m = make_movie(rate_kBps=RATE, length=12000, divide=False)
+    plan = plan_disc_split([m], CAP)
+    assert len(plan) >= 2
+    # first cut is an exact time cut at capacity
+    assert abs(plan[0][0][2] - SECS_PER_DISC) < 1.0
+
+
+def test_segments_cover_whole_movie(make_movie):
+    m = make_movie(rate_kBps=RATE, length=12000, divide=False)
+    plan = plan_disc_split([m], CAP)
+    # reconstruct covered span from (start, duration) segments
+    covered = 0.0
+    for disc in plan:
+        movie, start, duration = disc[0]
+        end = movie.original_length if duration == 0 else start + duration
+        assert start == covered  # contiguous, no gaps/overlaps
+        covered = end
+    assert covered == m.original_length
+
+
+def test_menu_reservation_on_first_disc(make_movie):
+    m = make_movie(rate_kBps=RATE, length=3000)
+    # reserve a chunk on disc 1; small movie still fits
+    plan = plan_disc_split([m], CAP, first_disc_reserved_kbytes=500_000)
+    assert len(plan) == 1
+
+
+def test_empty_project_returns_one_empty_disc():
+    plan = plan_disc_split([], CAP)
+    assert plan == [[]]
+
+
+def test_zero_rate_does_not_crash(make_movie):
+    m = make_movie(rate_kBps=0, length=3600)
+    plan = plan_disc_split([m], CAP)
+    assert len(plan) >= 1
+
+
+# --- chapter helpers ---
+
+def test_chapter_seconds_auto_and_manual(make_movie):
+    m = make_movie(length=1000, chapters_step=5, manual="2:30,9:00")
+    # auto: 300, 600, 900 (< length-4); manual: 150 (2:30), 540 (9:00)
+    assert _movie_chapter_seconds(m) == [150, 300, 540, 600, 900]
+
+
+def test_chapter_seconds_disabled(make_movie):
+    m = make_movie(length=1000, divide=False)
+    assert _movie_chapter_seconds(m) == []
+
+
+def test_nearest_chapter_before():
+    assert _nearest_chapter_before([300, 600, 900], 0, 700) == 600
+    assert _nearest_chapter_before([300, 600, 900], 0, 250) is None
+    assert _nearest_chapter_before([300, 600, 900], 600, 1000) == 900