fix(app): gate one-Tab message focus behind an experiment (#2762)

fluxer_app/src/features/messaging/utils/MessageNodeSelectors.ts

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+export const CHANNEL_MESSAGE_ID_PREFIX = 'chat-messages';
+
+export const escapeSelectorValue = (value: string): string =>
+	value.replace(/[\\"]/gu, '\\$&').replace(/[\n\r\f]/gu, (char) => `\\${char.charCodeAt(0).toString(16)} `);
+
+export const getMessageSelector = (channelId?: string, messageId?: string): string => {
+	const channelSelector = channelId ? `[data-channel-id="${escapeSelectorValue(channelId)}"]` : '[data-channel-id]';
+	const messageSelector = messageId ? `[data-message-id="${escapeSelectorValue(messageId)}"]` : '[data-message-id]';
+	return `${channelSelector}${messageSelector}`;
+};
+
+export const findMessageElement = (
+	doc: Document | null | undefined,
+	viewport: HTMLElement | null | undefined,
+	channelId: string,
+	messageId: string,
+): HTMLElement | null => {
+	const byId = doc?.getElementById(`${CHANNEL_MESSAGE_ID_PREFIX}-${channelId}-${messageId}`) ?? null;
+	if (byId != null) {
+		return byId as HTMLElement;
+	}
+	return viewport?.querySelector<HTMLElement>(getMessageSelector(channelId, messageId)) ?? null;
+};