dayHeaderContent assumed arg.date is always the real column date. It is in
the time-grid views, but FullCalendar v6 fills it from an internal reference
week (1970-01-04..10) for dayGridMonth headers, so the month header read a
fixed "Mo 05.01. ... So 04.01." regardless of the visible month. Body dates
were correct; only the header row was wrong.
Interpretation (flagged as ambiguous): a month-view column header labels seven
generic weekday columns shared by every week in the grid -- it has no single
date, so forcing one in is wrong by construction rather than just mis-computed.
Month view now renders the localized weekday alone, which is also FC's own
default there; timeGridWeek keeps weekday + DD.MM. since each column really is
one date.
Branches on view.type === 'dayGridMonth' exactly, not a dayGrid prefix:
dayGridWeek/dayGridDay do have real per-column dates and a prefix match would
break them if either is ever added.
Extracted to an exported formatDayHeader so it is testable without mounting
the page; the test mounts a real FullCalendar in both views, so an upgrade
that changes the arg.date contract fails rather than silently regresses.
FullCalendar dependency untouched.
Refs testplan REPORT.md #10 (Part 8, S9).
Paul Nothaft
76a1453fa7