A chevron in the tab strip drops the band, leaving the strip; persisted, so it survives a reload. Clicking a tab expands again, since a collapsed strip whose tabs do nothing is a trap. Word puts that chevron in the band's corner. It rides the strip so the band keeps its full width: a flex row can only reserve a column, never a corner. Skipped: Word floats the band over the document until the next click lands in the text.