QueryPlanViewer's step column takes the label width and the table
scrolls sideways past its least width; TableStructureEditor's rows
stretch to a least width that scrolls sideways; RedisKeyBrowser's
columns wrap below each other inside 280px. The form's rules are pure,
parse_port and ready, with unit tests; gpui tests cover saving, a new
connection refilling the form, another engine bringing its port, and a
press on a row's button leaving the row alone.