perf(app): stop unbounded WASM heap growth and idle-CPU churn (#1721)

fluxer_app/src/features/channel/state/ChannelDisplayNameTracking.ts

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+export interface TrackedRecipientSnapshot {
+	readonly recipientIds: ReadonlyArray<string>;
+}
+
+export interface TrackedRecipientUser {
+	readonly username: string;
+	readonly globalName?: string | null;
+}
+
+/**
+ * Builds the MobX tracking key for group-DM display names.
+ *
+ * Only the recipients of tracked group DMs are read, so the observing reaction
+ * depends on those users alone rather than on the whole (unbounded) user cache.
+ * Reading a recipient that is not cached yet still registers a dependency on
+ * that key, so a user arriving later re-triggers the reaction.
+ *
+ * Kept as a standalone function rather than a class method because
+ * makeAutoObservable converts methods into actions, and actions run untracked.
+ */
+export const trackedRecipientNameKey = (
+	snapshots: ReadonlyMap<string, TrackedRecipientSnapshot>,
+	users: Record<string, TrackedRecipientUser | undefined>,
+): string => {
+	const parts: Array<[string, string, string]> = [];
+	for (const snapshot of snapshots.values()) {
+		for (const recipientId of snapshot.recipientIds) {
+			const user = users[recipientId];
+			parts.push([recipientId, user?.username ?? '', user?.globalName ?? '']);
+		}
+	}
+	return JSON.stringify(parts);
+};