A table whose <thead> uses [colspan] to group columns under a shared header.

ⓘ Notice A [colspan]-ed cell never pins, even when it is a row’s first child — otherwise it would overlap the columns beneath it.

ⓘ Notice A row’s first column can only pin if it is a <th>, because that is explicit definition for cell in a column that labels it’s rows.

Scroll the table horizontally to confirm.