This behavior is partly "by design". The current behavior is optimized for scenarios where the table (from which content got fostered) is not edited. We want to prevent corruption (loss of information OR duplication of content) in those scenarios.

The wt2html parse assigns zero-width dsr info to fostered content. This works well in selser because when it encounters these nodes, it just emits an empty string and the table after it emits the original content (as long as it was not edited).

But, if the table is edited, the fostered content node before it goes to '', and the table is serialized normally. Since the table html no longer has the fostered content there, the table doesn't emit the fosterable content either, and effectively, that is lost.

The solution to this problem would be to fix our DOMDiff to set edit-flags of fostered content nodes to be the same as edit-flags of the table from which they were fostered.

(In reply to Roan Kattouw from comment #0)

User-facing example: go to
https://www.mediawiki.org/wiki/User:Roan_Kattouw_%28WMF%29?veaction=edit

Sorry, I meant https://www.mediawiki.org/wiki/User:Roan_Kattouw_%28WMF%29/Fostered_categories?veaction=edit

Still exists and reproducible as below.

[subbu@earth:~/work/wmf/parsoid] echo '<table><tr><td>Hello</td></tr>[[Category:Foo]]</table>' > /tmp/wt
[subbu@earth:~/work/wmf/parsoid] php bin/parse.php < /tmp/wt > /tmp/old.html
[subbu@earth:~/work/wmf/parsoid] sed 's/Hello/Hello there/g;' < /tmp/old.html > /tmp/new.html
[subbu@earth:~/work/wmf/parsoid] php bin/parse.php --html2wt < /tmp/new.html 
[[Category:Foo]]
<table><tr><td>Hello there</td></tr></table>

[subbu@earth:~/work/wmf/parsoid] php bin/parse.php --selser --oldtextfile /tmp/wt --oldhtmlfile /tmp/old.html < /tmp/new.html 
<table><tr><td>Hello there</td></tr></table>