argus's review (reproduced, not speculative): .ui-table-scroll sets
overflow-x: auto with overflow-y left unset. Per the CSS overflow
spec, an axis left visible computes to auto too once the other axis
isn't visible -- so this box silently clips vertically as well as
horizontally. The popover was position: absolute; top: 100% under its
header <th>, itself inside .ui-table-scroll -- on a table shorter than
header-plus-popover (a small hive's roster, or any table narrowed by
an existing filter), the popover got cut off at the scroll box's own
bottom edge, with a stray vertical scrollbar as the visible symptom.
Computes the popover's viewport position from the anchor <th>'s own
getBoundingClientRect() and renders it via a portal onto
document.body, position: fixed -- escapes .ui-table-scroll's clip the
same way any position: fixed element escapes an ancestor's overflow
(neither .ui-table-scroll nor .ui-table establishes a new containing
block). Recomputes on scroll (capture-phase window listener, the
standard technique for detecting scroll on a nested scroll container
without binding to every ancestor by hand) and resize while open, so
the popover stays anchored rather than only positioning once at click
time. createPortal comes from preact/compat, already resolvable
through the existing preact dependency -- no new package.json entry.
Verified against argus's own repro shape: a 1-row table, scripted
click on the filter icon (same real-DOM-event technique as this PR's
first round), screenshot shows the popover rendering fully rather than
clipped, no stray scrollbar.
63f3afae3e