feat: screenshot capture messenger

[jxiwang]

Summary

Extracts cross-window messenger functionality from plugin-autocapture-browser into a shared module in analytics-core, and adds background capture support that both autocapture and session-replay can use through a single shared messenger instance.

Problem

Previously, plugin-autocapture-browser owned its own WindowMessenger class, which handled all cross-window postMessage communication (visual tagging, ping/pong, etc.). Background capture is a new feature that needs to be available to both plugin-autocapture-browser and session-replay-browser. Without a shared messenger, each plugin would attach its own message listener and load its own copy of external scripts, causing duplicate listeners, duplicate script loads, and double notifications.

Solution

New messenger/ module in analytics-core:

• BaseWindowMessenger — Generic, feature-agnostic cross-window messenger using postMessage. Handles message routing, request/response correlation (with timeouts), action handler dispatch, and idempotent script loading. The class is not exported as a value; consumers must use the singleton factory.
• getOrCreateWindowMessenger() — Singleton factory stored on globalScope. A branded property (__AMPLITUDE_MESSENGER_INSTANCE__) is used instead of instanceof for reliable cross-bundle identity checks. This ensures that even when autocapture and session-replay are loaded as separate bundles, they share the same messenger instance and single message listener.
• enableBackgroundCapture(messenger) — Composable enabler function that registers background-capture action handlers on a messenger instance. Idempotent via a brand key — safe to call from both autocapture and session-replay; only the first call takes effect.
• Shared constants — Origin URLs (AMPLITUDE_ORIGIN, AMPLITUDE_ORIGIN_EU, AMPLITUDE_ORIGIN_STAGING) and the background capture script URL are now defined in analytics-core and re-exported from plugin-autocapture-browser for backwards compatibility.

Refactored plugin-autocapture-browser:

• The old WindowMessenger class is replaced by an enableVisualTagging(messenger, options) function that follows the same composable pattern — it registers visual-tagging-specific action handlers on the shared messenger.
• autocapture-plugin.ts now uses the singleton: getOrCreateWindowMessenger() → enableVisualTagging() → enableBackgroundCapture() → messenger.setup().
• The messenger option is removed from visualTaggingOptions since the singleton handles this.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?:

---

Note

Medium Risk
Touches cross-window messaging and dynamic script loading used by multiple plugins; mistakes could cause missed/duplicated postMessage handling or load failures across autocapture/session replay, though changes are guarded by singleton/idempotency and covered by tests.

Overview
Adds a new analytics-core messenger/ module, exposing getOrCreateWindowMessenger() (global singleton with branded identity), shared origin constants, and utilities for buffered action handling plus deduped external script loading.

Introduces enableBackgroundCapture() to register background-capture actions and load an external capture script once, then wires both plugin-autocapture-browser and session-replay-browser to use the shared messenger (including a refactor from WindowMessenger to enableVisualTagging() and removal of the visualTaggingOptions.messenger option). Tests are added/updated to validate singleton behavior, message buffering/idempotency, and background capture initialization/teardown.

^{Written by Cursor Bugbot for commit 676ae38. This will update automatically on new commits. Configure here.}

[daniel-graham-amplitude]

Lgtm.

[jxiwang]

bugbot run

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

[jxiwang]

bugbot run

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}