LuisSanchez commented on code in PR #36486:
URL: https://github.com/apache/superset/pull/36486#discussion_r2625182044


##########
superset-frontend/packages/superset-ui-core/src/components/TimezoneSelector/index.tsx:
##########
@@ -45,117 +63,160 @@ const offsetsToName = {
   '060': ['GMT Standard Time - London', 'British Summer Time'],
 };
 
-export type TimezoneSelectorProps = {
-  onTimezoneChange: (value: string) => void;
-  timezone?: string | null;
-  minWidth?: string;
-};
+let cachedTimezoneOptions: TimezoneOption[] | null = null;
+let computePromise: Promise<TimezoneOption[]> | null = null;
+
+function getOffsetKey(timezoneName: string): string {
+  return (
+    JANUARY_REF.tz(timezoneName).utcOffset().toString() +
+    JULY_REF.tz(timezoneName).utcOffset().toString()
+  );
+}
+
+function getTimezoneDisplayName(
+  timezoneName: string,
+  currentDate: ReturnType<typeof extendedDayjs>,
+): string {
+  const offsetKey = getOffsetKey(timezoneName);
+  const dateInZone = currentDate.tz(timezoneName);
+  const isDSTActive = isDST(dateInZone, timezoneName);
+  const namePair = offsetsToName[offsetKey];
+  return namePair ? (isDSTActive ? namePair[1] : namePair[0]) : timezoneName;
+}
+
+function computeTimezoneOptions(): TimezoneOption[] {
+  const currentDate = extendedDayjs(new Date());
+  const allZones = Intl.supportedValuesOf('timeZone');
+  const seenLabels = new Set<string>();
+  const options: TimezoneOption[] = [];
+
+  for (const zone of allZones) {
+    const offsetKey = getOffsetKey(zone);
+    const displayName = getTimezoneDisplayName(zone, currentDate);
+    const offset = extendedDayjs.tz(currentDate, zone).format('Z');
+    const label = `GMT ${offset} (${displayName})`;
+
+    if (!seenLabels.has(label)) {
+      seenLabels.add(label);
+      options.push({
+        label,
+        value: zone,
+        offsets: offsetKey,
+        timezoneName: zone,
+      });
+    }
+  }
+
+  // Pre-sort timezone options by time offset
+  options.sort(
+    (a, b) =>
+      extendedDayjs.tz(currentDate, a.timezoneName).utcOffset() -
+      extendedDayjs.tz(currentDate, b.timezoneName).utcOffset(),
+  );
+
+  cachedTimezoneOptions = options;
+  return options;
+}
+
+function getTimezoneOptionsAsync(): Promise<TimezoneOption[]> {
+  if (cachedTimezoneOptions) {
+    return Promise.resolve(cachedTimezoneOptions);
+  }
+
+  if (computePromise) {
+    return computePromise;
+  }
+
+  // Use queueMicrotask for better performance than setTimeout(0)
+  // Falls back to setTimeout for older browsers
+  computePromise = new Promise<TimezoneOption[]>(resolve => {
+    if (typeof queueMicrotask === 'function') {
+      queueMicrotask(() => {
+        resolve(computeTimezoneOptions());
+      });
+    } else {
+      setTimeout(() => {
+        resolve(computeTimezoneOptions());
+      }, 0);
+    }
+  });
+
+  return computePromise;
+}
+
+function findMatchingTimezone(
+  timezone: string | null | undefined,
+  options: TimezoneOption[],
+): string {
+  const targetTimezone = timezone || extendedDayjs.tz.guess();
+  const targetOffsetKey = getOffsetKey(targetTimezone);
+  let fallbackValue: string | undefined;
+
+  for (const option of options) {
+    if (
+      option.offsets === targetOffsetKey &&
+      option.timezoneName === targetTimezone
+    ) {
+      return option.value;
+    }
+    if (!fallbackValue && option.offsets === targetOffsetKey) {
+      fallbackValue = option.value;
+    }
+  }
+
+  return fallbackValue || DEFAULT_TIMEZONE.value;
+}
 
 export default function TimezoneSelector({
   onTimezoneChange,
   timezone,
-  minWidth = MIN_SELECT_WIDTH, // smallest size for current values
+  minWidth = MIN_SELECT_WIDTH,
+  placeholder,
   ...rest
 }: TimezoneSelectorProps) {
-  const { TIMEZONE_OPTIONS, TIMEZONE_OPTIONS_SORT_COMPARATOR, validTimezone } =
-    useMemo(() => {
-      const currentDate = extendedDayjs();
-      const JANUARY = extendedDayjs.tz('2021-01-01');
-      const JULY = extendedDayjs.tz('2021-07-01');
-
-      const getOffsetKey = (name: string) =>
-        JANUARY.tz(name).utcOffset().toString() +
-        JULY.tz(name).utcOffset().toString();
-
-      const getTimezoneName = (name: string) => {
-        const offsets = getOffsetKey(name);
-        return (
-          (isDST(currentDate.tz(name), name)
-            ? offsetsToName[offsets as keyof typeof offsetsToName]?.[1]
-            : offsetsToName[offsets as keyof typeof offsetsToName]?.[0]) || 
name
-        );
-      };
-
-      // TODO: remove this ts-ignore when typescript is upgraded to 5.1
-      // @ts-ignore
-      const ALL_ZONES: string[] = Intl.supportedValuesOf('timeZone');
-
-      const labels = new Set<string>();
-      const TIMEZONE_OPTIONS = ALL_ZONES.map(zone => {
-        const label = `GMT ${extendedDayjs
-          .tz(currentDate, zone)
-          .format('Z')} (${getTimezoneName(zone)})`;
-
-        if (labels.has(label)) {
-          return null; // Skip duplicates
-        }
-        labels.add(label);
-        return {
-          label,
-          value: zone,
-          offsets: getOffsetKey(zone),
-          timezoneName: zone,
-        };
-      }).filter(Boolean) as {
-        label: string;
-        value: string;
-        offsets: string;
-        timezoneName: string;
-      }[];
-
-      const TIMEZONE_OPTIONS_SORT_COMPARATOR = (
-        a: (typeof TIMEZONE_OPTIONS)[number],
-        b: (typeof TIMEZONE_OPTIONS)[number],
-      ) =>
-        extendedDayjs.tz(currentDate, a.timezoneName).utcOffset() -
-        extendedDayjs.tz(currentDate, b.timezoneName).utcOffset();
-
-      // pre-sort timezone options by time offset
-      TIMEZONE_OPTIONS.sort(TIMEZONE_OPTIONS_SORT_COMPARATOR);
-
-      const matchTimezoneToOptions = (timezone: string) => {
-        const offsetKey = getOffsetKey(timezone);
-        let fallbackValue: string | undefined;
-
-        for (const option of TIMEZONE_OPTIONS) {
-          if (
-            option.offsets === offsetKey &&
-            option.timezoneName === timezone
-          ) {
-            return option.value;
-          }
-          if (!fallbackValue && option.offsets === offsetKey) {
-            fallbackValue = option.value;
-          }
-        }
-        return fallbackValue || DEFAULT_TIMEZONE.value;
-      };
-
-      const validTimezone = matchTimezoneToOptions(
-        timezone || extendedDayjs.tz.guess(),
-      );
-
-      return {
-        TIMEZONE_OPTIONS,
-        TIMEZONE_OPTIONS_SORT_COMPARATOR,
-        validTimezone,
-      };
-    }, [timezone]);
-
-  // force trigger a timezone update if provided `timezone` is not invalid
-  useEffect(() => {
-    if (validTimezone && timezone !== validTimezone) {
-      onTimezoneChange(validTimezone);
+  const [timezoneOptions, setTimezoneOptions] = useState<
+    TimezoneOption[] | null
+  >(cachedTimezoneOptions);
+  const [isLoadingOptions, setIsLoadingOptions] = useState(false);
+
+  const handleOpenChange = useCallback(
+    (isOpen: boolean) => {
+      if (isOpen && !timezoneOptions && !isLoadingOptions) {
+        setIsLoadingOptions(true);
+        getTimezoneOptionsAsync()
+          .then(setTimezoneOptions)
+          .finally(() => setIsLoadingOptions(false));
+      }
+    },
+    [timezoneOptions, isLoadingOptions],
+  );
+
+  const sortComparator = useMemo(() => {
+    if (!timezoneOptions) return undefined;
+    const currentDate = extendedDayjs();
+    return (a: TimezoneOption, b: TimezoneOption) =>
+      extendedDayjs.tz(currentDate, a.timezoneName).utcOffset() -
+      extendedDayjs.tz(currentDate, b.timezoneName).utcOffset();
+  }, [timezoneOptions]);

Review Comment:
   This is needed for the Selector component, does not apply.



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