Preserve unchanged habit renders and memoize calendar geometry

src/lib/dashboard.ts

@@ -1,7 +1,7 @@
 import type { HabitService } from "../habits/service";
 import type { Schedule } from "../habits/contracts";
 
-export type TodayHabit = ReturnType<HabitService["day"]> & { calendarRevision?: number };
+export type TodayHabit = ReturnType<HabitService["day"]>;
 export type TodayResponse = {
   date: string;
   timezone: string;
@@ -46,3 +46,15 @@ export function formatTrackingDate(date: string) {
     timeZone: "UTC",
   });
 }
+
+/** Preserve unchanged row references when a background refresh returns new JSON objects. */
+export function reconcileToday(current: TodayResponse | null, next: TodayResponse): TodayResponse {
+  if (!current || current.date !== next.date || current.timezone !== next.timezone) return next;
+  const previous = new Map(current.habits.map(habit => [habit.habitId, habit]));
+  const habits = next.habits.map(habit => {
+    const old = previous.get(habit.habitId);
+    return old && JSON.stringify(old) === JSON.stringify(habit) ? old : habit;
+  });
+  if (current.due === next.due && current.completed === next.completed && habits.length === current.habits.length && habits.every((habit, index) => habit === current.habits[index])) return current;
+  return { ...next, habits };
+}