Currently, the "Optimize MP4" option is only available as a manual step from the right-click/Utils menu. I'd like to request an option to have Mp3tag apply this optimization automatically every time tags are saved, rather than requiring a separate manual step afterwards.
When editing MP4/M4A tags, Mp3tag reuses the existing moov/free atom space where possible to avoid a full-file rewrite. But this means the size of the leftover free padding atom after a save depends on the file's prior edit history, not just its current tag content. Two files with logically identical tags can end up with different padding, and therefore different file sizes and different checksums, purely as an artifact of how many times each file was previously saved.
For example If I use "Tag Copy"/"Tag Paste" from an existing tagged M4A file, then adjust individual tags after pasting, such as track number or title, even though the resulting tag content across episodes is equivalent in structure, the files end up with different amounts of padding (in my case, 3979 bytes vs 4096 bytes) simply because of differing save
history. This makes byte-for-byte comparison / deduplication of otherwise-identical files impossible without a manual "Optimize MP4" pass afterwards.
Could we have an an "Always optimize MP4 atoms on save" option (default off, for anyone relying on the current fast in-place-write behaviour) that makes Mp3tag perform the equivalent of "Utils → Optimize MP4" automatically as part of every tag save for MP4/M4A files? This would give MP4 files parity with the padding control ID3v2/MP3 already has via the Options → Tags → Mp3 padding setting, without requiring users to remember a separate manual cleanup step.