Steps to replicate the issue (include links if applicable):

• Create a page with some text. Make a couple edits.
• Copy paste the contents of the first page to a new second page. Save.
• Make a couple edits.
• Go to Special:MergeHistory and merge the first page into the second page.
• View the history of the second page.

What happens?:

• The edit that created the second page has a positive size diff (e.g. +1234 bytes)

What should have happened instead?:

• The edit that created the second page should have a zero size diff (e.g. 0 bytes). This is because no text was added between these two diffs. The wikitext is identical.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

• The fix is to have Special:MergeHistory recalculate the size of the diff after the seam.
• This has likely created a bunch of database corruption. If worth fixing, would probably need a maintenance script that checks the entries in the merge log, then targets those pages for recalculation.