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mkvpropedit crashes if no seek element present and no space to add one #1516
A file created by x264 doesn't usually have a seek element present. Neither is there a void element before the first cluster.
When the user does something with the file that adds a level 1 element without moving the existing level 1 elements to the end of the file then mkvpropedit will abort with the following message:
If the segment or track headers are modified then the problem doesn't occur as those elements are relocated to the end of the file first leaving a void and enough space before the first cluster to fit a new meta seek element. But adding attachments or chapters will write those attachments/chapters at the end, and then there's no space left at the beginning for writing that meta seek element.
The GUI's header editor should not be affected by this issue as it only modifies the track headers and the segment info. However, the chapter editor is affected if you save chapters to such a x264 file.
See #1513 where this issue was discovered first.