feat: scroll the calendar month view continuously #759

The month view showed one month at a time and only the arrows moved it.
With the new "Free scrolling" setting (on by default, the webmail's
`calendarFreeScroll`) it is one continuous list of weeks, as in the
webmail (15a5497a, ae403b06):

- the month under the upper part of the grid is drawn at full strength,
  names the title and is where the arrows step from; the first day of
  each month carries the month's name
- reaching the top or the bottom adds weeks there and loads only their
  events; weeks added above keep the rows on screen in place
- rows have a fixed height and snap to whole weeks, so the list stays
  windowed and navigation lands exactly on the month

Turning the setting off brings back the paged month and a fixed 30-day
agenda. The week view still pages.

src/components/calendar/MonthScrollView.tsx

@@ -0,0 +1,265 @@
+import React from 'react';
+import {
+  FlatList,
+  View,
+  type ListRenderItemInfo,
+  type NativeScrollEvent,
+  type NativeSyntheticEvent,
+} from 'react-native';
+import type { Calendar, CalendarEvent } from '../../api/types';
+import {
+  buildEventDayIndex,
+  dayKey,
+  type EventDayIndex,
+  type TimeFormat,
+} from '../../lib/calendar-utils';
+import type { CalendarFocus, DayRange } from '../../lib/calendar-scroll-window';
+import {
+  dayIndexIn,
+  monthFocusRow,
+  monthKeyOf,
+  monthMask,
+  sampledRow,
+  weekDays,
+  windowWeekStarts,
+} from '../../lib/calendar-month-scroll';
+import { useCalendarLocale } from '../../lib/calendar-locale';
+import {
+  MONTH_ROW_HEIGHT,
+  MONTH_ROW_HEIGHT_CHIPS,
+  MonthWeekRow,
+  MonthWeekdayHeader,
+  useMonthStyles,
+} from './MonthView';
+
+type WeekStart = 0 | 1 | 6;
+
+/** Rows the month viewport shows, as one month grid used to. */
+const VISIBLE_ROWS = 6;
+/** Navigation closer than this many rows scrolls smoothly; further jumps. */
+const SMOOTH_SCROLL_ROWS = 12;
+
+interface MonthScrollViewProps {
+  /** The day the user navigated to; its month's first week is scrolled to the top. */
+  focus: CalendarFocus;
+  /** The loaded window: whole weeks, one row each. */
+  window: DayRange;
+  selectedDate: Date;
+  events: CalendarEvent[];
+  calendars: Calendar[];
+  eventsByDay?: EventDayIndex;
+  weekStartsOn?: WeekStart;
+  showWeekNumbers?: boolean;
+  showTimeInMonthView?: boolean;
+  timeFormat?: TimeFormat;
+  /** Widen the window at the top; omitted once the limit is reached. */
+  onExtendStart?: () => void;
+  /** Widen the window at the bottom; omitted once the limit is reached. */
+  onExtendEnd?: () => void;
+  /** Reports a day of the month in view whenever that month changes. */
+  onVisibleDateChange?: (date: Date) => void;
+  onSelectDate: (date: Date) => void;
+  onLongPressDate?: (date: Date) => void;
+}
+
+/**
+ * Freely scrolling month (#759, webmail 15a5497a): the window's weeks as one
+ * continuous list of fixed-height rows. The month under the upper part of
+ * the viewport is the one in focus: its days are drawn at full strength,
+ * the title follows it, and the first of every month carries its name.
+ * Reaching either end widens the window; rows added above keep what is on
+ * screen in place. The screen remounts the list (keyed by the window) when
+ * navigation starts a fresh window.
+ */
+function MonthScrollViewInner({
+  focus,
+  window,
+  selectedDate,
+  events,
+  calendars,
+  eventsByDay,
+  weekStartsOn = 0,
+  showWeekNumbers = false,
+  showTimeInMonthView = false,
+  timeFormat,
+  onExtendStart,
+  onExtendEnd,
+  onVisibleDateChange,
+  onSelectDate,
+  onLongPressDate,
+}: MonthScrollViewProps) {
+  const styles = useMonthStyles();
+  const { locale } = useCalendarLocale();
+  const index = React.useMemo(
+    () => eventsByDay ?? buildEventDayIndex(events),
+    [eventsByDay, events],
+  );
+  const opts = React.useMemo(() => ({ weekStartsOn }), [weekStartsOn]);
+
+  // One row per week of the window. A week keeps its array across window
+  // changes, so growing the window doesn't re-render the rows already there.
+  const weekCacheRef = React.useRef(new Map<string, Date[]>());
+  const weeks = React.useMemo(() => {
+    const previous = weekCacheRef.current;
+    const next = new Map<string, Date[]>();
+    const out = windowWeekStarts(window).map((start) => {
+      const key = dayKey(start);
+      const days = previous.get(key) ?? weekDays(start);
+      next.set(key, days);
+      return days;
+    });
+    weekCacheRef.current = next;
+    return out;
+  }, [window]);
+  const weeksRef = React.useRef(weeks);
+  weeksRef.current = weeks;
+  const rowHeight = showTimeInMonthView ? MONTH_ROW_HEIGHT_CHIPS : MONTH_ROW_HEIGHT;
+  const viewportHeight = rowHeight * VISIBLE_ROWS;
+
+  // The month in focus: set by navigation, then by what scrolls under the
+  // sample line.
+  const [activeMonth, setActiveMonth] = React.useState(() => monthKeyOf(focus.date));
+  const activeMonthRef = React.useRef(activeMonth);
+  const onVisibleRef = React.useRef(onVisibleDateChange);
+  onVisibleRef.current = onVisibleDateChange;
+  const offsetRef = React.useRef(0);
+  // Row at the top of the viewport; a change of row height (the "show time"
+  // setting) remounts the list there.
+  const topRowRef = React.useRef<number | null>(null);
+
+  const handleScroll = React.useCallback(
+    (e: NativeSyntheticEvent<NativeScrollEvent>) => {
+      const offset = e.nativeEvent.contentOffset.y;
+      offsetRef.current = offset;
+      topRowRef.current = Math.round(offset / rowHeight);
+      const list = weeksRef.current;
+      const row = sampledRow(offset, viewportHeight, rowHeight, list.length);
+      const days = list[row];
+      if (!days) return;
+      // Mid-week decides which month a row belongs to.
+      const mid = days[3];
+      const key = monthKeyOf(mid);
+      if (key === activeMonthRef.current) return;
+      activeMonthRef.current = key;
+      setActiveMonth(key);
+      onVisibleRef.current?.(mid);
+    },
+    [viewportHeight, rowHeight],
+  );
+
+  // Navigation inside the window scrolls the focused month's first week to
+  // the top. The list mounts there already, so its own nonce needs nothing.
+  const listRef = React.useRef<FlatList<Date[]>>(null);
+  const handledNonceRef = React.useRef(focus.nonce);
+  const initialRef = React.useRef<{ rowHeight: number; row: number } | null>(null);
+  if (initialRef.current === null || initialRef.current.rowHeight !== rowHeight) {
+    initialRef.current = {
+      rowHeight,
+      row: topRowRef.current ?? monthFocusRow(window, focus.date, weeks.length, opts),
+    };
+  }
+  React.useEffect(() => {
+    if (handledNonceRef.current === focus.nonce) return;
+    handledNonceRef.current = focus.nonce;
+    const row = monthFocusRow(window, focus.date, weeks.length, opts);
+    const currentRow = Math.round(offsetRef.current / rowHeight);
+    const key = monthKeyOf(focus.date);
+    activeMonthRef.current = key;
+    setActiveMonth(key);
+    listRef.current?.scrollToIndex({
+      index: row,
+      animated: Math.abs(row - currentRow) <= SMOOTH_SCROLL_ROWS,
+    });
+  }, [focus, window, weeks.length, opts, rowHeight]);
+
+  const selectedKey = dayKey(selectedDate);
+  const todayKey = dayKey(new Date());
+
+  const renderItem = React.useCallback(
+    ({ item: days }: ListRenderItemInfo<Date[]>) => {
+      return (
+        <MonthWeekRow
+          days={days}
+          activeMask={monthMask(days, activeMonth)}
+          selectedIndex={dayIndexIn(days, selectedDate)}
+          todayIndex={dayIndexIn(days, new Date())}
+          index={index}
+          calendars={calendars}
+          weekStartsOn={weekStartsOn}
+          showWeekNumbers={showWeekNumbers}
+          showTimeInMonthView={showTimeInMonthView}
+          timeFormat={timeFormat}
+          labelMonths
+          height={rowHeight}
+          locale={locale}
+          styles={styles}
+          onSelectDate={onSelectDate}
+          onLongPressDate={onLongPressDate}
+        />
+      );
+    },
+    [
+      activeMonth,
+      selectedDate,
+      index,
+      calendars,
+      weekStartsOn,
+      showWeekNumbers,
+      showTimeInMonthView,
+      timeFormat,
+      rowHeight,
+      locale,
+      styles,
+      onSelectDate,
+      onLongPressDate,
+    ],
+  );
+
+  const getItemLayout = React.useCallback(
+    (_: ArrayLike<Date[]> | null | undefined, i: number) => ({
+      length: rowHeight,
+      offset: rowHeight * i,
+      index: i,
+    }),
+    [rowHeight],
+  );
+
+  return (
+    <View style={styles.grid}>
+      <MonthWeekdayHeader weekStartsOn={weekStartsOn} showWeekNumbers={showWeekNumbers} styles={styles} />
+      <FlatList
+        key={rowHeight}
+        ref={listRef}
+        style={{ height: viewportHeight }}
+        data={weeks}
+        keyExtractor={weekKey}
+        renderItem={renderItem}
+        extraData={`${activeMonth}|${selectedKey}|${todayKey}`}
+        getItemLayout={getItemLayout}
+        initialScrollIndex={Math.min(initialRef.current.row, weeks.length - 1)}
+        // Weeks added above keep the rows on screen where they are.
+        maintainVisibleContentPosition={{ minIndexForVisible: 0 }}
+        // Half the grid: the window opens with a month of room either side,
+        // so this doesn't fire while the list settles on the focus.
+        onStartReached={onExtendStart}
+        onStartReachedThreshold={0.5}
+        onEndReached={onExtendEnd}
+        onEndReachedThreshold={0.5}
+        onScroll={handleScroll}
+        scrollEventThrottle={32}
+        snapToInterval={rowHeight}
+        decelerationRate="fast"
+        showsVerticalScrollIndicator={false}
+        initialNumToRender={VISIBLE_ROWS + 2}
+        maxToRenderPerBatch={VISIBLE_ROWS}
+        windowSize={5}
+      />
+    </View>
+  );
+}
+
+export const MonthScrollView = React.memo(MonthScrollViewInner);
+
+function weekKey(days: Date[]): string {
+  return dayKey(days[0]);
+}

src/components/calendar/MonthView.tsx

@@ -7,11 +7,9 @@ import {
   format,
   getISOWeek,
   getWeek,
-  isSameDay,
-  isSameMonth,
-  isToday,
   startOfMonth,
   startOfWeek,
+  type Locale,
 } from 'date-fns';
 import type { Calendar, CalendarEvent } from '../../api/types';
 import { componentSizes, radius, spacing, typography, type ThemePalette } from '../../theme/tokens';
@@ -25,10 +23,16 @@ import {
   type EventDayIndex,
   type TimeFormat,
 } from '../../lib/calendar-utils';
+import { dayIndexIn, monthKeyOf, monthMask } from '../../lib/calendar-month-scroll';
 import { useCalendarLocale } from '../../lib/calendar-locale';
 
 type WeekStart = 0 | 1 | 6;
 
+// Height of a week row with dots, and with event chips (#666). The freely
+// scrolling month uses them as fixed row heights.
+export const MONTH_ROW_HEIGHT = 54;
+export const MONTH_ROW_HEIGHT_CHIPS = 74;
+
 interface MonthViewProps {
   currentDate: Date;
   selectedDate: Date;
@@ -52,6 +56,175 @@ export function weekNumberFor(date: Date, weekStartsOn: WeekStart): number {
   return getWeek(date, { weekStartsOn });
 }
 
+export type MonthStyles = ReturnType<typeof makeStyles>;
+
+export function useMonthStyles(): MonthStyles {
+  const c = useColors();
+  return React.useMemo(() => makeStyles(c), [c]);
+}
+
+export function MonthWeekdayHeader({
+  weekStartsOn,
+  showWeekNumbers,
+  styles,
+}: {
+  weekStartsOn: WeekStart;
+  showWeekNumbers: boolean;
+  styles: MonthStyles;
+}) {
+  const { locale } = useCalendarLocale();
+  const labels = React.useMemo(() => {
+    const start = startOfWeek(new Date(), { weekStartsOn });
+    return Array.from({ length: 7 }, (_, i) => format(addDays(start, i), 'EEEEE', { locale }));
+  }, [weekStartsOn, locale]);
+  return (
+    <View style={styles.weekdayRow}>
+      {showWeekNumbers && <Text style={styles.weekNumberLabel}>#</Text>}
+      {labels.map((wd, i) => (
+        <Text key={i} style={styles.weekdayLabel}>{wd}</Text>
+      ))}
+    </View>
+  );
+}
+
+export interface MonthWeekRowProps {
+  days: Date[];
+  /** Bit i set: day i belongs to the month in focus (others are muted). */
+  activeMask: number;
+  /** Index of the selected / today's day in this row, or -1. */
+  selectedIndex: number;
+  todayIndex: number;
+  index: EventDayIndex;
+  calendars: Calendar[];
+  weekStartsOn: WeekStart;
+  showWeekNumbers: boolean;
+  showTimeInMonthView: boolean;
+  timeFormat?: TimeFormat;
+  /** Mark the first day of a month with the month's name (continuous scrolling). */
+  labelMonths?: boolean;
+  /** Fixed row height (continuous scrolling); natural height otherwise. */
+  height?: number;
+  locale: Locale;
+  styles: MonthStyles;
+  onSelectDate: (date: Date) => void;
+  onLongPressDate?: (date: Date) => void;
+}
+
+function MonthWeekRowInner({
+  days,
+  activeMask,
+  selectedIndex,
+  todayIndex,
+  index,
+  calendars,
+  weekStartsOn,
+  showWeekNumbers,
+  showTimeInMonthView,
+  timeFormat,
+  labelMonths = false,
+  height,
+  locale,
+  styles,
+  onSelectDate,
+  onLongPressDate,
+}: MonthWeekRowProps) {
+  return (
+    <View style={[styles.weekRow, height !== undefined && { height, overflow: 'hidden' }]}>
+      {showWeekNumbers && (
+        <Text style={styles.weekNumberCell}>{weekNumberFor(days[0], weekStartsOn)}</Text>
+      )}
+      {days.map((d, i) => {
+        const sameMonth = (activeMask & (1 << i)) !== 0;
+        const today = i === todayIndex;
+        const selected = i === selectedIndex;
+        const dayEvents = eventsOnDayFromIndex(index, d);
+        const MAX_DOTS = dayEvents.length > 3 ? 2 : 3;
+        const MAX_CHIPS = dayEvents.length > 2 ? 1 : 2;
+        const maxVisible = showTimeInMonthView ? MAX_CHIPS : MAX_DOTS;
+        const visible = dayEvents.slice(0, maxVisible);
+        const overflow = Math.max(0, dayEvents.length - maxVisible);
+        const monthLabel = labelMonths && d.getDate() === 1 ? format(d, 'MMM', { locale }) : null;
+
+        return (
+          <Pressable
+            key={i}
+            style={[styles.dayCell, showTimeInMonthView && styles.dayCellTall]}
+            onPress={() => onSelectDate(d)}
+            onLongPress={onLongPressDate ? () => onLongPressDate(d) : undefined}
+          >
+            <View style={[
+              styles.dayNumber,
+              today && styles.todayCircle,
+              selected && !today && styles.selectedCircle,
+            ]}>
+              {monthLabel && (
+                <Text
+                  numberOfLines={1}
+                  style={[
+                    styles.monthLabel,
+                    !sameMonth && styles.dayTextMuted,
+                    today && styles.todayText,
+                    selected && !today && styles.selectedText,
+                  ]}
+                >
+                  {monthLabel}
+                </Text>
+              )}
+              <Text style={[
+                styles.dayText,
+                monthLabel !== null && styles.dayTextUnderLabel,
+                !sameMonth && styles.dayTextMuted,
+                today && styles.todayText,
+                selected && !today && styles.selectedText,
+              ]}>
+                {format(d, 'd')}
+              </Text>
+            </View>
+            {dayEvents.length > 0 && !showTimeInMonthView && (
+              <View style={styles.dotsRow}>
+                {visible.map((event, idx) => (
+                  <View
+                    key={`${event.id}-${idx}`}
+                    style={[styles.dot, { backgroundColor: getEventColor(event, calendars) }]}
+                  />
+                ))}
+                {overflow > 0 && (
+                  <Text style={styles.overflowText}>+{overflow}</Text>
+                )}
+              </View>
+            )}
+            {dayEvents.length > 0 && showTimeInMonthView && (
+              <View style={styles.chipsCol}>
+                {visible.map((event, idx) => {
+                  const color = getEventColor(event, calendars);
+                  const cancelled = event.status === 'cancelled';
+                  return (
+                    <View key={`${event.id}-${idx}`} style={[styles.chip, { backgroundColor: color }]}>
+                      <Text
+                        style={[styles.chipText, cancelled && styles.chipTextCancelled]}
+                        numberOfLines={1}
+                      >
+                        {event.showWithoutTime
+                          ? (event.title || '')
+                          : `${format(getEventStartDate(event), timePattern(timeFormat), { locale })} ${event.title || ''}`}
+                      </Text>
+                    </View>
+                  );
+                })}
+                {overflow > 0 && (
+                  <Text style={styles.overflowText}>+{overflow}</Text>
+                )}
+              </View>
+            )}
+          </Pressable>
+        );
+      })}
+    </View>
+  );
+}
+
+export const MonthWeekRow = React.memo(MonthWeekRowInner);
+
 function MonthViewInner({
   currentDate,
   selectedDate,
@@ -65,130 +238,50 @@ function MonthViewInner({
   onSelectDate,
   onLongPressDate,
 }: MonthViewProps) {
-  const c = useColors();
-  const styles = React.useMemo(() => makeStyles(c), [c]);
+  const styles = useMonthStyles();
   const { locale } = useCalendarLocale();
-  const weekdayLabels = React.useMemo(() => {
-    const start = startOfWeek(new Date(), { weekStartsOn });
-    return Array.from({ length: 7 }, (_, i) => format(addDays(start, i), 'EEEEE', { locale }));
-  }, [weekStartsOn, locale]);
   const index = React.useMemo(
     () => eventsByDay ?? buildEventDayIndex(events),
     [eventsByDay, events],
   );
-  const days = React.useMemo(() => {
-    const monthStart = startOfMonth(currentDate);
-    const monthEnd = endOfMonth(currentDate);
-    const calStart = startOfWeek(monthStart, { weekStartsOn });
-    const calEnd = endOfWeek(monthEnd, { weekStartsOn });
-    const result: Date[] = [];
+
+  // The month grid as rows of 7 days.
+  const rows = React.useMemo(() => {
+    const calStart = startOfWeek(startOfMonth(currentDate), { weekStartsOn });
+    const calEnd = endOfWeek(endOfMonth(currentDate), { weekStartsOn });
+    const out: Date[][] = [];
     let day = calStart;
     while (day <= calEnd) {
-      result.push(day);
-      day = addDays(day, 1);
+      out.push(Array.from({ length: 7 }, (_, i) => addDays(day, i)));
+      day = addDays(day, 7);
     }
-    return result;
-  }, [currentDate, weekStartsOn]);
-
-  // Group days into rows of 7 so we can prepend a week-number column.
-  const rows = React.useMemo(() => {
-    const out: Date[][] = [];
-    for (let i = 0; i < days.length; i += 7) out.push(days.slice(i, i + 7));
     return out;
-  }, [days]);
-
-  const renderDay = (d: Date, key: React.Key, flexLayout = false) => {
-    const sameMonth = isSameMonth(d, currentDate);
-    const today = isToday(d);
-    const selected = isSameDay(d, selectedDate);
-    const dayEvents = eventsOnDayFromIndex(index, d);
-    const MAX_DOTS = dayEvents.length > 3 ? 2 : 3;
-    const MAX_CHIPS = dayEvents.length > 2 ? 1 : 2;
-    const maxVisible = showTimeInMonthView ? MAX_CHIPS : MAX_DOTS;
-    const visible = dayEvents.slice(0, maxVisible);
-    const overflow = Math.max(0, dayEvents.length - maxVisible);
-
-    return (
-      <Pressable
-        key={key}
-        style={[styles.dayCell, flexLayout && styles.dayCellFlex, showTimeInMonthView && styles.dayCellTall]}
-        onPress={() => onSelectDate(d)}
-        onLongPress={onLongPressDate ? () => onLongPressDate(d) : undefined}
-      >
-        <View style={[
-          styles.dayNumber,
-          today && styles.todayCircle,
-          selected && !today && styles.selectedCircle,
-        ]}>
-          <Text style={[
-            styles.dayText,
-            !sameMonth && styles.dayTextMuted,
-            today && styles.todayText,
-            selected && !today && styles.selectedText,
-          ]}>
-            {format(d, 'd')}
-          </Text>
-        </View>
-        {dayEvents.length > 0 && !showTimeInMonthView && (
-          <View style={styles.dotsRow}>
-            {visible.map((event, idx) => (
-              <View
-                key={`${event.id}-${idx}`}
-                style={[styles.dot, { backgroundColor: getEventColor(event, calendars) }]}
-              />
-            ))}
-            {overflow > 0 && (
-              <Text style={styles.overflowText}>+{overflow}</Text>
-            )}
-          </View>
-        )}
-        {dayEvents.length > 0 && showTimeInMonthView && (
-          <View style={styles.chipsCol}>
-            {visible.map((event, idx) => {
-              const color = getEventColor(event, calendars);
-              const cancelled = event.status === 'cancelled';
-              return (
-                <View key={`${event.id}-${idx}`} style={[styles.chip, { backgroundColor: color }]}>
-                  <Text
-                    style={[styles.chipText, cancelled && styles.chipTextCancelled]}
-                    numberOfLines={1}
-                  >
-                    {event.showWithoutTime
-                      ? (event.title || '')
-                      : `${format(getEventStartDate(event), timePattern(timeFormat), { locale })} ${event.title || ''}`}
-                  </Text>
-                </View>
-              );
-            })}
-            {overflow > 0 && (
-              <Text style={styles.overflowText}>+{overflow}</Text>
-            )}
-          </View>
-        )}
-      </Pressable>
-    );
-  };
+  }, [currentDate, weekStartsOn]);
+  const activeMonth = monthKeyOf(currentDate);
+  const today = new Date();
 
   return (
     <View style={styles.grid}>
-      <View style={styles.weekdayRow}>
-        {showWeekNumbers && <Text style={styles.weekNumberLabel}>#</Text>}
-        {weekdayLabels.map((wd, i) => (
-          <Text key={i} style={styles.weekdayLabel}>{wd}</Text>
-        ))}
-      </View>
-      {showWeekNumbers ? (
-        rows.map((row, rowIdx) => (
-          <View key={rowIdx} style={styles.weekRow}>
-            <Text style={styles.weekNumberCell}>{weekNumberFor(row[0], weekStartsOn)}</Text>
-            {row.map((d, i) => renderDay(d, `${rowIdx}-${i}`, true))}
-          </View>
-        ))
-      ) : (
-        <View style={styles.daysGrid}>
-          {days.map((d, i) => renderDay(d, i))}
-        </View>
-      )}
+      <MonthWeekdayHeader weekStartsOn={weekStartsOn} showWeekNumbers={showWeekNumbers} styles={styles} />
+      {rows.map((days) => (
+        <MonthWeekRow
+          key={days[0].toISOString()}
+          days={days}
+          activeMask={monthMask(days, activeMonth)}
+          selectedIndex={dayIndexIn(days, selectedDate)}
+          todayIndex={dayIndexIn(days, today)}
+          index={index}
+          calendars={calendars}
+          weekStartsOn={weekStartsOn}
+          showWeekNumbers={showWeekNumbers}
+          showTimeInMonthView={showTimeInMonthView}
+          timeFormat={timeFormat}
+          locale={locale}
+          styles={styles}
+          onSelectDate={onSelectDate}
+          onLongPressDate={onLongPressDate}
+        />
+      ))}
     </View>
   );
 }
@@ -205,7 +298,6 @@ function makeStyles(c: ThemePalette) {
     ...typography.small,
     color: c.textMuted,
   },
-  daysGrid: { flexDirection: 'row', flexWrap: 'wrap' },
   weekRow: { flexDirection: 'row', alignItems: 'flex-start' },
   weekNumberLabel: {
     width: 24,
@@ -220,9 +312,8 @@ function makeStyles(c: ThemePalette) {
     color: c.textMuted,
     paddingVertical: 4,
   },
-  dayCell: { width: '14.28%', alignItems: 'center', paddingVertical: 4 },
-  dayCellFlex: { width: undefined, flex: 1 },
-  dayCellTall: { minHeight: 74, paddingHorizontal: 1 },
+  dayCell: { flex: 1, alignItems: 'center', paddingVertical: 4 },
+  dayCellTall: { minHeight: MONTH_ROW_HEIGHT_CHIPS, paddingHorizontal: 1 },
   dayNumber: {
     width: 36,
     height: 36,
@@ -231,6 +322,9 @@ function makeStyles(c: ThemePalette) {
     justifyContent: 'center',
   },
   dayText: { ...typography.body, color: c.text },
+  // Day 1 under its month's name: both fit the 36px circle.
+  dayTextUnderLabel: { lineHeight: 16 },
+  monthLabel: { fontSize: 8, lineHeight: 10, fontWeight: '600', color: c.textSecondary, textTransform: 'uppercase' },
   dayTextMuted: { color: c.textMuted },
   todayCircle: { backgroundColor: c.primary },
   todayText: { color: c.textInverse, fontWeight: '700' },

src/components/settings/CalendarSettings.tsx

@@ -41,6 +41,7 @@ export function CalendarSettings() {
   const timeZone = useSettingsStore((s) => s.calendarTimeZone);
   const showTimeInMonth = useSettingsStore((s) => s.calendarShowTimeInMonth);
   const showWeekNumbers = useSettingsStore((s) => s.calendarShowWeekNumbers);
+  const freeScroll = useSettingsStore((s) => s.calendarFreeScroll);
   const birthdayCal = useSettingsStore((s) => s.showBirthdayCalendar);
   const tasksEnabled = useSettingsStore((s) => s.enableCalendarTasks);
   const showTasksOnCal = useSettingsStore((s) => s.showTasksOnCalendar);
@@ -140,6 +141,19 @@ export function CalendarSettings() {
         />
       </SettingItem>
 
+      <SettingItem
+        label={t('calendar.settings.calendar_free_scroll', 'Free scrolling')}
+        description={t(
+          'calendar.settings.calendar_free_scroll_desc',
+          'Scroll continuously through months, weeks and days instead of one period at a time',
+        )}
+      >
+        <ToggleSwitch
+          checked={freeScroll}
+          onChange={(v) => update('calendarFreeScroll', v)}
+        />
+      </SettingItem>
+
       <SettingItem
         label={t('calendar.settings.show_birthday_calendar', 'Contact birthday calendar')}
         description={t(

src/lib/__tests__/calendar-month-scroll.test.ts

@@ -0,0 +1,117 @@
+import { describe, it, expect } from 'vitest';
+import { dayKey } from '../calendar-utils';
+import {
+  computeScrollWindow,
+  freshScrollWindowState,
+  growScrollWindow,
+} from '../calendar-scroll-window';
+import {
+  dayIndexIn,
+  monthFocusRow,
+  monthKeyOf,
+  monthMask,
+  sampledRow,
+  weekDays,
+  windowWeekStarts,
+} from '../calendar-month-scroll';
+
+const opts = { weekStartsOn: 1 as const };
+
+describe('windowWeekStarts', () => {
+  it('has one row per week of the window, each starting on the first weekday', () => {
+    const win = computeScrollWindow(freshScrollWindowState('month', new Date(2026, 8, 9)), opts);
+    const weeks = windowWeekStarts(win);
+    // 2026-07-27 .. 2026-11-08 is 15 weeks.
+    expect(weeks).toHaveLength(15);
+    expect(dayKey(weeks[0])).toBe('2026-07-27');
+    expect(dayKey(weeks[14])).toBe('2026-11-02');
+    expect(weeks.every((w) => w.getDay() === 1)).toBe(true);
+  });
+
+  it('keeps whole days across the DST change', () => {
+    const win = { start: new Date(2026, 9, 19), end: new Date(2026, 10, 1) };
+    const weeks = windowWeekStarts(win);
+    expect(weeks.map(dayKey)).toEqual(['2026-10-19', '2026-10-26']);
+    expect(weeks[1].getHours()).toBe(0);
+  });
+
+  it('adds rows above when the window grows into the past', () => {
+    const state = freshScrollWindowState('month', new Date(2026, 8, 9));
+    const before = windowWeekStarts(computeScrollWindow(state, opts));
+    const after = windowWeekStarts(computeScrollWindow(growScrollWindow(state, 'before'), opts));
+    const added = after.length - before.length;
+    expect(added).toBeGreaterThanOrEqual(4);
+    // The rows that were there keep their dates, shifted down by `added`.
+    expect(after.slice(added).map(dayKey)).toEqual(before.map(dayKey));
+  });
+});
+
+describe('weekDays', () => {
+  it('lists the seven days of a week', () => {
+    expect(weekDays(new Date(2026, 8, 28)).map(dayKey)).toEqual([
+      '2026-09-28', '2026-09-29', '2026-09-30', '2026-10-01', '2026-10-02', '2026-10-03', '2026-10-04',
+    ]);
+  });
+});
+
+describe('monthFocusRow', () => {
+  const win = computeScrollWindow(
+    growScrollWindow(freshScrollWindowState('month', new Date(2026, 8, 9)), 'before'),
+    opts,
+  );
+  const count = windowWeekStarts(win).length;
+
+  it('is the row of the week that starts the month grid', () => {
+    const row = monthFocusRow(win, new Date(2026, 9, 20), count, opts);
+    // October 2026's grid starts on Monday 2026-09-28.
+    expect(dayKey(windowWeekStarts(win)[row])).toBe('2026-09-28');
+  });
+
+  it('stays inside the list', () => {
+    expect(monthFocusRow(win, new Date(2020, 0, 1), count, opts)).toBe(0);
+    expect(monthFocusRow(win, new Date(2030, 0, 1), count, opts)).toBe(count - 1);
+  });
+});
+
+describe('sampledRow', () => {
+  it('picks the row under 40% of the viewport', () => {
+    // 6 rows of 54px: the sample line is 129.6px below the top.
+    expect(sampledRow(0, 324, 54, 20)).toBe(2);
+    expect(sampledRow(54 * 5, 324, 54, 20)).toBe(7);
+  });
+
+  it('clamps to the rows there are', () => {
+    expect(sampledRow(-50, 324, 54, 20)).toBe(1);
+    expect(sampledRow(100000, 324, 54, 20)).toBe(19);
+    expect(sampledRow(0, 324, 54, 0)).toBe(0);
+  });
+});
+
+describe('monthMask / monthKeyOf', () => {
+  it('marks the days of the month in focus', () => {
+    const days = weekDays(new Date(2026, 8, 28)); // Sep 28 .. Oct 4
+    expect(monthMask(days, monthKeyOf(new Date(2026, 8, 1)))).toBe(0b0000111);
+    expect(monthMask(days, monthKeyOf(new Date(2026, 9, 1)))).toBe(0b1111000);
+    expect(monthMask(days, monthKeyOf(new Date(2026, 10, 1)))).toBe(0);
+  });
+
+  it('tells months of different years apart', () => {
+    expect(monthKeyOf(new Date(2026, 0, 1))).not.toBe(monthKeyOf(new Date(2027, 0, 1)));
+    expect(monthKeyOf(new Date(2026, 11, 31)) + 1).toBe(monthKeyOf(new Date(2027, 0, 1)));
+  });
+});
+
+describe('dayIndexIn', () => {
+  const days = weekDays(new Date(2026, 8, 28));
+
+  it('finds a day in the row regardless of the time', () => {
+    expect(dayIndexIn(days, new Date(2026, 9, 1, 23, 30))).toBe(3);
+    expect(dayIndexIn(days, new Date(2026, 8, 28))).toBe(0);
+  });
+
+  it('is -1 outside the row', () => {
+    expect(dayIndexIn(days, new Date(2026, 9, 5))).toBe(-1);
+    expect(dayIndexIn(days, new Date(2026, 8, 27))).toBe(-1);
+    expect(dayIndexIn([], new Date())).toBe(-1);
+  });
+});

src/lib/__tests__/calendar-scroll-window.test.ts

@@ -47,9 +47,10 @@ describe('baseRange', () => {
 });
 
 describe('computeScrollWindow', () => {
-  it('starts as the base range plus the initial days after it, snapped to whole weeks', () => {
+  it('starts as the base range plus a step either side, snapped to whole weeks', () => {
     const win = computeScrollWindow(freshScrollWindowState('month', new Date(2026, 8, 9)), opts);
-    expect(local(win.start)).toBe('2026-08-31');
+    // 2026-08-31 - 30 days = 2026-08-01 (Saturday) -> start of that week
+    expect(local(win.start)).toBe('2026-07-27');
     // 2026-10-04 + 30 days = 2026-11-03 (Tuesday) -> end of that week
     expect(local(win.end)).toBe('2026-11-08');
     expect(win.canExtendStart).toBe(true);
@@ -76,15 +77,15 @@ describe('computeScrollWindow', () => {
 
   it('does not snap the day view to weeks', () => {
     const win = computeScrollWindow(freshScrollWindowState('day', new Date(2026, 8, 9)), opts);
-    expect(local(win.start)).toBe('2026-09-09');
+    expect(local(win.start)).toBe('2026-08-10');
     expect(local(win.end)).toBe('2026-10-09');
   });
 
   it('snaps a grown week window to whole weeks', () => {
     const state = growScrollWindow(freshScrollWindowState('week', new Date(2026, 8, 9)), 'before');
     const win = computeScrollWindow(state, opts);
-    // 2026-09-07 - 30 days = 2026-08-08 (Saturday) -> Monday 2026-08-03
-    expect(local(win.start)).toBe('2026-08-03');
+    // 2026-09-07 - 60 days = 2026-07-09 (Thursday) -> Monday 2026-07-06
+    expect(local(win.start)).toBe('2026-07-06');
     expect(win.start.getDay()).toBe(1);
     expect(win.end.getDay()).toBe(0);
   });
@@ -137,8 +138,8 @@ describe('normalizeScrollWindowState / scrollWindowContains', () => {
     const win = computeScrollWindow(freshScrollWindowState('month', new Date(2026, 8, 9)), opts);
     expect(scrollWindowContains(win, 'month', new Date(2026, 9, 15), opts)).toBe(true); // October grid ends Nov 1
     expect(scrollWindowContains(win, 'month', new Date(2026, 10, 15), opts)).toBe(false); // November grid ends Dec 6
-    expect(scrollWindowContains(win, 'week', new Date(2026, 7, 31), opts)).toBe(true);
-    expect(scrollWindowContains(win, 'week', new Date(2026, 7, 30), opts)).toBe(false);
+    expect(scrollWindowContains(win, 'week', new Date(2026, 6, 27), opts)).toBe(true); // window starts Jul 27
+    expect(scrollWindowContains(win, 'week', new Date(2026, 6, 26), opts)).toBe(false);
   });
 
   it('keys the anchor by calendar day', () => {

src/lib/calendar-month-scroll.ts

@@ -0,0 +1,73 @@
+import { addDays, differenceInCalendarDays } from 'date-fns';
+import { baseRange, type DayRange, type ScrollWindowOptions } from './calendar-scroll-window';
+
+/**
+ * Geometry of the freely scrolling month view (#759, webmail
+ * calendar-month-view): one fixed-height row per week of the window.
+ */
+
+/** Fraction of the viewport height at which the "current month" is sampled. */
+export const VISIBLE_MONTH_SAMPLE = 0.4;
+
+/** The first day of every week in the window (which is made of whole weeks). */
+export function windowWeekStarts(window: DayRange): Date[] {
+  const count = Math.floor(differenceInCalendarDays(window.end, window.start) / 7) + 1;
+  const weeks: Date[] = [];
+  for (let i = 0; i < count; i++) weeks.push(addDays(window.start, i * 7));
+  return weeks;
+}
+
+/** The seven days of the week starting at `weekStart`. */
+export function weekDays(weekStart: Date): Date[] {
+  return Array.from({ length: 7 }, (_, i) => addDays(weekStart, i));
+}
+
+/** Row of the week that starts `date`'s month grid: where navigation puts the top. */
+export function monthFocusRow(
+  window: DayRange,
+  date: Date,
+  rowCount: number,
+  opts: ScrollWindowOptions,
+): number {
+  const gridStart = baseRange('month', date, opts).start;
+  const row = Math.floor(differenceInCalendarDays(gridStart, window.start) / 7);
+  return Math.max(0, Math.min(rowCount - 1, row));
+}
+
+/** Row under the sample line for a scroll offset. */
+export function sampledRow(
+  offset: number,
+  viewportHeight: number,
+  rowHeight: number,
+  rowCount: number,
+  fraction = VISIBLE_MONTH_SAMPLE,
+): number {
+  if (rowCount <= 0 || rowHeight <= 0) return 0;
+  const row = Math.floor((offset + viewportHeight * fraction) / rowHeight);
+  return Math.max(0, Math.min(rowCount - 1, row));
+}
+
+/** Year and month as one comparable number. */
+export function monthKeyOf(date: Date): number {
+  return date.getFullYear() * 12 + date.getMonth();
+}
+
+/**
+ * Bit i is set when day i of the week belongs to the month `monthKey`. Rows
+ * compare this instead of the month itself, so moving to another month only
+ * re-renders the rows whose shading changes.
+ */
+export function monthMask(days: Date[], monthKey: number): number {
+  let mask = 0;
+  days.forEach((day, i) => {
+    if (monthKeyOf(day) === monthKey) mask |= 1 << i;
+  });
+  return mask;
+}
+
+/** Index of `date` in `days` (same calendar day), or -1. */
+export function dayIndexIn(days: Date[], date: Date): number {
+  if (days.length === 0) return -1;
+  const diff = differenceInCalendarDays(date, days[0]);
+  return diff >= 0 && diff < days.length ? diff : -1;
+}

src/lib/calendar-scroll-window.ts

@@ -71,13 +71,26 @@ const INITIAL_AFTER: Record<ScrollViewMode, number> = {
   day: SCROLL_WINDOW_STEP,
 };
 
+/**
+ * Days loaded before the base range at first. The grids open with a step
+ * of room above / to the left, so the list isn't mounted right at its start
+ * edge (where rows would be prepended while it is still settling on the
+ * focus). The agenda starts at its anchor and loads the past on request.
+ */
+const INITIAL_BEFORE: Record<ScrollViewMode, number> = {
+  month: SCROLL_WINDOW_STEP,
+  agenda: 0,
+  week: SCROLL_WINDOW_STEP,
+  day: SCROLL_WINDOW_STEP,
+};
+
 export function parseDayKey(key: string): Date {
   const [y, m, d] = key.split('-').map(Number);
   return new Date(y, m - 1, d);
 }
 
 export function freshScrollWindowState(mode: ScrollViewMode, anchor: Date): ScrollWindowState {
-  return { mode, anchorKey: dayKey(anchor), before: 0, after: INITIAL_AFTER[mode] };
+  return { mode, anchorKey: dayKey(anchor), before: INITIAL_BEFORE[mode], after: INITIAL_AFTER[mode] };
 }
 
 /**

src/screens/CalendarScreen.tsx

@@ -40,6 +40,7 @@ import { Button } from '../components';
 import { useCalendarStore } from '../stores/calendar-store';
 import { useSettingsStore } from '../stores/settings-store';
 import { MonthView } from '../components/calendar/MonthView';
+import { MonthScrollView } from '../components/calendar/MonthScrollView';
 import { WeekView } from '../components/calendar/WeekView';
 import { AgendaView } from '../components/calendar/AgendaView';
 import { EventCard } from '../components/calendar/EventCard';
@@ -154,6 +155,7 @@ export default function CalendarScreen() {
   const showTasksOnCalendar = useSettingsStore((s) => s.showTasksOnCalendar);
   const calendarFirstDayOfWeek = useSettingsStore((s) => s.calendarFirstDayOfWeek);
   const calendarShowWeekNumbers = useSettingsStore((s) => s.calendarShowWeekNumbers);
+  const calendarFreeScroll = useSettingsStore((s) => s.calendarFreeScroll);
   const calendarTimeFormat = useSettingsStore((s) => s.calendarTimeFormat);
   const showBirthdayCalendar = useSettingsStore((s) => s.showBirthdayCalendar);
   const enableCalendarTasks = useSettingsStore((s) => s.enableCalendarTasks);
@@ -172,10 +174,12 @@ export default function CalendarScreen() {
   const [viewMode, setViewMode] = React.useState<ViewMode>(initialViewMode);
 
   // Scroll window (#759, webmail calendar-app): every view shows a window of
-  // days around the day the user navigated to (the "focus"). The agenda
-  // scrolls freely: reaching an edge widens that side and only the new days
-  // are fetched. Month and week show exactly one period. Navigation moves the
-  // focus and, when that leaves the window, starts a fresh window there.
+  // days around the day the user navigated to (the "focus"). With free
+  // scrolling (the default) the agenda and the month scroll continuously:
+  // reaching an edge widens that side and only the new days are fetched.
+  // Otherwise, and in the week view, a view shows exactly one period.
+  // Navigation moves the focus and, when that leaves the window, starts a
+  // fresh window there.
   const windowOptions = React.useMemo<ScrollWindowOptions>(
     () => ({ weekStartsOn: calendarFirstDayOfWeek }),
     [calendarFirstDayOfWeek],
@@ -186,7 +190,7 @@ export default function CalendarScreen() {
   const [windowState, setWindowState] = React.useState<ScrollWindowState>(
     () => freshScrollWindowState(initialViewMode, new Date()),
   );
-  const freeScroll = viewMode === 'agenda';
+  const freeScroll = calendarFreeScroll && (viewMode === 'agenda' || viewMode === 'month');
   const focusKey = dayKey(focus.date);
   const activeWindowState = React.useMemo(
     () =>
@@ -403,20 +407,16 @@ export default function CalendarScreen() {
   }, [syncDueSubscriptions]);
 
   // Load the window's events: one fetch at a time, and only the days not
-  // loaded yet (calendar-store extendRange). `loadingEdge` is the side an
-  // extension asked for, until the wider window is in.
+  // loaded yet (calendar-store extendRange). `loadingEdge` is the side the
+  // last extension asked for, until the wider window is in.
   const [rangeLoading, setRangeLoading] = React.useState(false);
   const [loadingEdge, setLoadingEdge] = React.useState<'start' | 'end' | null>(null);
-  const loadingEdgeRef = React.useRef<'start' | 'end' | null>(null);
   const [rangeLoader] = React.useState(() =>
     createRangeLoader(
       (range) => useCalendarStore.getState().extendRange(range.after, range.before),
       (busy) => {
         setRangeLoading(busy);
-        if (!busy) {
-          loadingEdgeRef.current = null;
-          setLoadingEdge(null);
-        }
+        if (!busy) setLoadingEdge(null);
       },
     ),
   );
@@ -440,12 +440,11 @@ export default function CalendarScreen() {
     [viewMode, windowOptions],
   );
 
+  // The grids add their rows for the wider window right away (the events
+  // follow); the agenda waits for one extension to load before the next.
   const extendWindow = React.useCallback(
     (side: 'before' | 'after') => {
-      if (loadingEdgeRef.current) return;
-      const edge = side === 'before' ? 'start' : 'end';
-      loadingEdgeRef.current = edge;
-      setLoadingEdge(edge);
+      setLoadingEdge(side === 'before' ? 'start' : 'end');
       setWindowState((prev) =>
         growScrollWindow(normalizeScrollWindowState(prev, viewMode, focus.date), side),
       );
@@ -957,7 +956,27 @@ export default function CalendarScreen() {
       )}
 
       <View style={styles.content}>
-        {viewMode === 'month' && (
+        {viewMode === 'month' && freeScroll && (
+          <MonthScrollView
+            key={windowKey}
+            focus={focus}
+            window={scrollWindow}
+            onExtendStart={scrollWindow.canExtendStart ? extendWindowStart : undefined}
+            onExtendEnd={scrollWindow.canExtendEnd ? extendWindowEnd : undefined}
+            onVisibleDateChange={setVisibleDate}
+            selectedDate={selectedDate}
+            events={events}
+            eventsByDay={eventsByDay}
+            calendars={calendars}
+            weekStartsOn={calendarFirstDayOfWeek}
+            showWeekNumbers={calendarShowWeekNumbers}
+            showTimeInMonthView={calendarShowTimeInMonth}
+            timeFormat={calendarTimeFormat}
+            onSelectDate={handleSelectDate}
+            onLongPressDate={openCreate}
+          />
+        )}
+        {viewMode === 'month' && !freeScroll && (
           <MonthView
             currentDate={focus.date}
             selectedDate={selectedDate}

src/stores/settings-store.ts

@@ -232,6 +232,9 @@ interface PersistedSettings {
   calendarTimeFormat: TimeFormat;
   calendarShowTimeInMonth: boolean;
   calendarShowWeekNumbers: boolean;
+  // Scroll continuously through months, weeks and days (#759) instead of
+  // one period at a time. Same key as the webmail's setting.
+  calendarFreeScroll: boolean;
   calendarHoverPreview: CalendarHoverPreview;
   // IANA zone the calendar works in, or 'auto' to follow the device (#755).
   // Same key semantics as the webmail's `timeZone` setting.
@@ -392,6 +395,7 @@ const DEFAULT_PERSISTED: PersistedSettings = {
   calendarTimeFormat: '24h',
   calendarShowTimeInMonth: true,
   calendarShowWeekNumbers: false,
+  calendarFreeScroll: true,
   calendarHoverPreview: 'delay-500ms',
   calendarTimeZone: 'auto',
   showBirthdayCalendar: false,