When moving through string matches on a page, the foundStringMatchIndex is sometimes reset to 0. Seems to happen where two load chunks meet -- I don't know about WebKit's page loading strategy but I assume pages are rendered in chunks and the counter reset happens where I would guess the cutoffs for chunk loading are.
This is reproducible, for example, on the Google results page for "cheese" if you search for the string "cheese."
Created attachment 380241 [details]
Comment on attachment 380241 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=380241&action=review
radar # should be here if it exists ahead of time (see other patches on trac.webkit.org)
Created attachment 380267 [details]
Comment on attachment 380267 [details]
Clearing flags on attachment: 380267
Committed r250779: <https://trac.webkit.org/changeset/250779>
All reviewed patches have been landed. Closing bug.