Put back the allow attribute my last commit orphaned

Inserting the title-elision helper directly above render_titlebar pushed
that function away from its own #[allow(clippy::too_many_arguments)], which
then applied to a const and left the function warning again. Third time I
have done this to an attribute in this tree; the pattern is inserting at a
"just before this function" anchor without checking what sits immediately
above it.

Also `"x".repeat(20_000)` in the new test, which clippy asked for.

Clippy is clean again - and it was not when I said it was in the previous
commit: the check printed its own "ok" line unconditionally, so three real
warnings scrolled past above it.

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