[mvanhorn]

Calendar: optional ISO week numbers on the month grid

The month view and the sidebar mini-calendar can show the ISO week number of each row. The setting is off by default, so the 7-column layout is unchanged until it is turned on.

What changes

• Show week numbers sits with the other layout toggles in the account menu. When it is on, a gutter of week numbers appears to the left of the month grid and of the sidebar mini-calendar.
• Each number is the ISO week of the Thursday in that 7-day row, so it stays the same whether the week starts on Sunday or Monday. Clicking a number opens that week. The week-view title gains · Week N beside the date range.
• The year view is left as it is: those mini-months are too tight for another column.
• The setting is stored with the rest of the user's preferences. A value that is not the boolean true is treated as off.

Tests

• isoWeekNumber / displayedWeekNumber cover week 1 spanning December, a 53-week year, a mid-year Thursday, Sunday-start vs Monday-start rows, and that the helpers do not mutate their input.
• Month and mini-calendar render tests cover the default 7-column layout, the numbers for July 2026, a Monday-start row, and a click opening that week.
• App tests cover the account-menu toggle, the week-view title with the setting on and off, and that clicking a month-grid number switches to week view without writing defaultView.
• Store load sanitizes a non-boolean showWeekNumbers back to false.
• npm test: 1769 passed, 3 skipped. npm run build succeeds.

Test plan

• cd src/calendar && npm ci && npm test && npm run build
• Month view, account menu, turn on Show week numbers; confirm a number appears on each row and the weekday headers still line up
• Click a week number and land on that week, with Week N in the title
• Turn on Start week on Monday and confirm the number on a given row is still the ISO week of that row's Thursday
• Reload and confirm the setting is still on; turn it off and confirm the gutter is gone

Not included

• Week numbers are not drawn on the year view.
• There is no separate numbering scheme besides ISO weeks.

Demo