Three related bugs in a horizontally scrolling data table with sticky (frozen) left columns and a sticky right "+" column, plus header popovers: (1) a border-left on a position:sticky cell in a border-collapse table is not visible until the table is scrolled to the end; (2) a closed popover list with a Tailwind `flex` class is rendered open inside the header cell; (3) keyboard focus lands on a header link half hidden under the sticky columns, even with scroll-padding on the scroll container.