Might be doable together with other text-based <input> types, see https://bugs.webkit.org/show_bug.cgi?id=54179
I don't think this is correct. <isindex> is speced to uses a non-shadow DOM.
Specifically, it is handled as a HTML parser "macro".
The prompt and hr are a parser macro; I assume Roland is referring to cleaning up the text input field itself.
Looking at IsIndexInputType, it looks like if TextFieldInputType uses the new shadow DOM model, there is nothing that needs to be done for isindex. So I think we should close this bug.
I also don't expect that we have to do anything special here, I added this bug mainly for bookkeeping reasons. However, i would prefer if we close only after the conversion for the other text types is actually done, as unforeseen gotchas lurk everywhere </pessimist>.
Looking with Dominic at the parser-generated DOM again, it's probably safe to already close the bug after all.