Fix FixInvalidItalicTags eating a character before a dangling begin tag#12589
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In the "two begin tags, one end tag" repair path, the near-end branch did Substring(0, lastIndex - 1) + endTag: it cut the character before the dangling begin tag and appended an unbalanced end tag, so "<i>ab</i>c<i>" became "<i>ab</i></i>" (the "c" was silently lost) and "<i>a</i><i>" became "<i>a</i</i>" (a corrupted tag). The other branch was already just removing the spurious begin tag in a roundabout way (lastIndex - 1 + endTag.Length == lastIndex + beginTag.Length), so both collapse into a single Remove that keeps all surrounding text. Runs as part of "Fix common errors" (fix invalid italic tags), so the old behavior destroyed subtitle text. Present since 2021, also in SE4. Unit tests added for the three corruption cases. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Found in a bug hunt and verified against the real library. In
HtmlUtil.FixInvalidItalicTags, the "two<i>, one</i>" repair path had an off-by-one in its near-end branch —Substring(0, lastIndex - 1) + endTagcut the character before the dangling begin tag and appended an unbalanced end tag:<i>a</i><i><i>a</i</i>(corrupted tag)<i>a</i><i>ab</i>c<i><i>ab</i></i>(cdeleted)<i>ab</i>c<i>Hello</i> d<i><i>Hello</i> </i>(ddeleted)<i>Hello</i> dThis runs inside "Fix common errors" → fix invalid italic tags, so the old behavior silently destroyed subtitle text. The bug has been there since 2021 and is also present in SE4.
The fix: the sibling branch was already removing exactly the spurious begin tag in a roundabout way (
lastIndex - 1 + endTag.Length==lastIndex + beginTag.Length), so both branches collapse into a singletext.Remove(lastIndex, beginTag.Length)that keeps all surrounding text.Unit tests added for the three corruption cases; the full LibSETests suite passes (502/502).
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