Preserve unchanged habit renders and memoize calendar geometry

src/pages/Home.tsx

@@ -2,7 +2,7 @@ import { CalendarCacheProvider } from "../components/CalendarCache";
 import { Archive, Plus, Settings2 } from "lucide-react";
 import { ArchivedHabits } from "../components/ArchivedHabits";
 import { AccountSettings } from "../components/AccountSettings";
-import { useCallback, useEffect, useRef, useState } from "react";
+import { memo, useCallback, useEffect, useRef, useState } from "react";
 import { useAuth } from "../components/AuthProvider";
 import { DiscordSignInButton } from "../components/DiscordSignInButton";
 import { PageLayout } from "../components/design-system/PageLayout";
@@ -17,7 +17,7 @@ import { Card, CardGrid } from "../components/design-system/Card";
 import { HabitChart } from "../components/design-system/HabitChart";
 import { HABIT_COLORS } from "../components/design-system/calendar-model";
 import { HabitForm } from "../components/HabitForm";
-import { habitRequest, scheduleLabel, type TodayHabit, type TodayResponse } from "../lib/dashboard";
+import { habitRequest, reconcileToday, scheduleLabel, type TodayHabit, type TodayResponse } from "../lib/dashboard";
 import { HabitHistory } from "../components/HabitHistory";
 import { WelcomePanel } from "../components/design-system/WelcomePanel";
 import type { PublicUser } from "../shared/user";
@@ -203,7 +203,7 @@ function Dashboard({ user, onExpired }: { user: PublicUser; onExpired: () => voi
     habitRequest<TodayResponse>("/today", { signal: controller.signal })
       .then((data) => {
         if (!controller.signal.aborted) {
-          setToday(data);
+          setToday(current => reconcileToday(current, data));
           setNeedsRefresh(false);
           setRevision((value) => value + 1);
         }
@@ -301,7 +301,7 @@ function Dashboard({ user, onExpired }: { user: PublicUser; onExpired: () => voi
       try {
         const refreshed = await habitRequest<TodayResponse>("/today");
         if (mounted.current) {
-          setToday(refreshed);
+          setToday(current => reconcileToday(current, refreshed));
           // Keep a habit reachable when an edit makes it unscheduled (including an empty routine).
           if (refreshed.habits.some(item => item.habitId === habit.habitId && !item.due)) setHabitView("all");
           setNeedsRefresh(false);
@@ -336,6 +336,13 @@ function Dashboard({ user, onExpired }: { user: PublicUser; onExpired: () => voi
     }
   }
 
+  const actions = useRef({ update, manage });
+  actions.current = { update, manage };
+  const updateHabit = useCallback((...args: Parameters<typeof update>) => actions.current.update(...args), []);
+  const manageHabit = useCallback((...args: Parameters<typeof manage>) => actions.current.manage(...args), []);
+  const refreshHistory = useCallback(() => setAttempt(value => value + 1), []);
+  const expire = useCallback(() => expired.current(), []);
+
   return (
     <>
       <WelcomePanel
@@ -458,10 +465,10 @@ function Dashboard({ user, onExpired }: { user: PublicUser; onExpired: () => voi
                     date={today.date}
                     revision={habit.calendarRevision ?? habitRevisions[habit.habitId] ?? 0}
                     disabled={busy || pendingHabits.has(habit.habitId) || loading || needsRefresh}
-                    onHistorySaved={() => setAttempt(value => value + 1)}
-                    onUpdate={update}
-                    onManage={manage}
-                    onExpired={() => expired.current()}
+                    onHistorySaved={refreshHistory}
+                    onUpdate={updateHabit}
+                    onManage={manageHabit}
+                    onExpired={expire}
                   />
                 ))}
               </div>
@@ -473,7 +480,7 @@ function Dashboard({ user, onExpired }: { user: PublicUser; onExpired: () => voi
   );
 }
 
-function SavedHabit({
+const SavedHabit = memo(function SavedHabit({
   habit,
   date,
   revision,
@@ -603,7 +610,7 @@ function SavedHabit({
       ) : undefined}
     </HabitHistory>
   );
-}
+});
 
 function SavedTask({
   task,