HandBrake · Primary-source explanation; examples are illustrative. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
Treat the detected borders as a proposal
HandBrake documents automatic cropping as an attempt to detect and remove black borders, and provides custom controls when the estimate is wrong. A source can include changing aspect ratios, dark scenery or useful information near an edge. Inspect what would actually be removed instead of assuming that the detected crop is always empty space. Choosing an output resolution limit is a separate decision from deciding which source pixels to retain.
Review different kinds of scene
For an original example, a permitted home film begins with a letterboxed introduction but later fills more of the frame. Check both portions in the preview before accepting the same top and bottom crop for the whole export. Include a scene with an edge title or scoreboard if one exists. Compare a conservative custom crop with the detected values while keeping the other picture settings unchanged, and note which content would be lost.
Preserve the framing decision with the source
Choose the crop which serves the intended output and review the completed file at the affected transitions. Enlarging the remaining picture cannot recover material already excluded from the encode. Keep the original available until framing, subtitles and important edge details have been checked. This is a source inspection workflow, not a rule that all borders should remain or disappear, and it does not claim that an automatic estimate was tested on a particular recording.