ContentDetector merge filter: one short scene early on changes where (or whether) a much later cut is reported

[sungbin1015]

I'm using ContentDetector with threshold=15 and min_scene_len of about 1s (default filter_mode) to split long sports broadcasts into shots. While testing on synthetic clips I noticed that a single short scene early in the video changes how a cut hundreds of frames later gets reported.

import cv2, numpy as np
from scenedetect import open_video, SceneManager, ContentDetector

def make(path, cuts, n=600):
    # 30 fps, alternating black/white shots, a hard cut at every frame in `cuts`
    w = cv2.VideoWriter(path, cv2.VideoWriter_fourcc(*"mp4v"), 30.0, (320, 180))
    for i in range(n):
        w.write(np.full((180, 320, 3), 255 * (sum(i >= c for c in cuts) % 2), np.uint8))
    w.release()

def detect(path):
    sm = SceneManager()
    sm.add_detector(ContentDetector(threshold=15, min_scene_len=30))
    sm.detect_scenes(open_video(path))
    return [start.frame_num for start, _ in sm.get_scene_list()[1:]]

for cuts in ([100, 400, 415], [100, 110, 400, 415], [100, 580], [100, 110, 580]):
    make("t.mp4", cuts)
    print(cuts, "->", detect("t.mp4"))

On 0.7.2 (same on 0.6.7 and 0.7.0, OpenCV 5.0.0):

[100, 400, 415] -> [100, 400]
[100, 110, 400, 415] -> [100, 415]
[100, 580] -> [100, 580]
[100, 110, 580] -> [100]

The 2nd and 4th clips only add a cut at frame 110, which is 290 / 470 frames before the cut I care about. With it, the 400/415 pair comes out at 415 instead of 400, and the cut at 580 (20 frames before the end) is gone entirely. With filter_mode=FlashFilter.Mode.SUPPRESS all four give 400 / 580 as I'd expect.

From reading _filter_merge, the 100→110 short scene sets _merge_triggered, and that only clears once _last_above - _merge_start >= min_scene_len, which a lone short scene never satisfies. So the filter stays armed through the quiet stretch, then holds the next real cut until another min_scene_len of quiet frames, and loses it if the video ends first. Your comment in #35 describes cuts being suppressed "until min-scene-len frames pass without a cut", so I expected it to reset somewhere in those 290 frames. #469 looks related, but there the difference came from changing min_scene_len, not from an earlier short scene with the same settings.

Is this the intended merge behaviour, or should the merge state clear after min_scene_len frames without a cut?