Tests: Add a test for seeking MatroskaDemuxer to EOS

Tests/LibMedia/CMakeLists.txt

@@ -5,6 +5,7 @@ set(TEST_SOURCES
     TestCICP.cpp
     TestH264Decode.cpp
     TestIncrementallyPopulatedStream.cpp
+    TestMatroskaDemuxer.cpp
     TestParseMatroska.cpp
     TestPlaybackStream.cpp
     TestVorbisDecode.cpp

Tests/LibMedia/TestMatroskaDemuxer.cpp

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2026, Gregory Bertilson <gregory@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/File.h>
+#include <LibMedia/Containers/Matroska/MatroskaDemuxer.h>
+#include <LibTest/TestCase.h>
+
+TEST_CASE(seek_past_eos)
+{
+    auto file = MUST(Core::File::open("./vfr.mkv"sv, Core::File::OpenMode::Read));
+    auto stream = Media::IncrementallyPopulatedStream::create_from_buffer(MUST(file->read_until_eof()));
+    auto demuxer = MUST(Media::Matroska::MatroskaDemuxer::from_stream(stream));
+
+    auto optional_track = MUST(demuxer->get_preferred_track_for_type(Media::TrackType::Video));
+    EXPECT(optional_track.has_value());
+    auto track = optional_track.release_value();
+    MUST(demuxer->create_context_for_track(track));
+
+    AK::Duration last_timestamp;
+    while (true) {
+        auto sample_result = demuxer->get_next_sample_for_track(track);
+        if (sample_result.is_error()) {
+            EXPECT_EQ(sample_result.error().category(), Media::DecoderErrorCategory::EndOfStream);
+            break;
+        }
+        last_timestamp = sample_result.release_value().timestamp();
+    }
+    EXPECT_EQ(last_timestamp, AK::Duration::from_milliseconds(30126));
+
+    auto seek_time = AK::Duration::from_milliseconds(31000);
+    MUST(demuxer->seek_to_most_recent_keyframe(track, seek_time, Media::DemuxerSeekOptions::None));
+    auto sample_after_seek = MUST(demuxer->get_next_sample_for_track(track));
+    EXPECT_EQ(sample_after_seek.timestamp(), AK::Duration::zero());
+}