Reduce RTC Polling

[the-hercules]

Closes #76631

Why?

High-level implementation overview

This update adds adaptive RTC polling for the HTTP polling transport using two signals:

1. Network Information API when available (navigator.connection.effectiveType)
2. Observed poll RTT fallback when that API is not available (Firefox, Safari, iOS browsers)

The goal is to reduce unnecessary polling on slower networks without changing the existing polling model or interfering with error backoff / background-tab behavior.

1. Network Information API path

For browsers that expose navigator.connection.effectiveType, we use it as an immediate hint for active-tab polling.

Implemented in:

• packages/sync/src/providers/http-polling/network-info.ts
• packages/sync/src/providers/http-polling/polling-manager.ts

What changed:

• Added feature detection for navigator.connection?.effectiveType
• Kept the existing multiplier logic:
  • 4g -> 1x
  • 3g -> 4x
  • 2g / slow-2g -> 10x
• Continued capping active-tab polling at 25000ms so awareness stays below the server timeout
• Kept support for the change event so pending polling can be rescheduled if the browser reports a new connection type mid-session

2. Cross-browser RTT fallback

For browsers that do not expose effectiveType, polling now adapts based on recent successful poll latency.

Implemented in:

• packages/sync/src/providers/http-polling/latency-info.ts
• packages/sync/src/providers/http-polling/polling-manager.ts

What changed:

• Added a small internal helper to track the last 3 successful poll durations
• Measured RTT around postSyncUpdate() in the polling loop
• Only recorded successful polls
• Used the average of the last 3 successful polls to choose a multiplier:
  • average < 500ms -> 1x
  • average >= 500ms and < 1500ms -> 4x
  • average >= 1500ms -> 10x
• Applied that multiplier to the existing base interval:
  • collaborators -> 1000ms
  • solo editing -> 4000ms
• Cleared RTT history when the last room unregisters so a new session starts cleanly

Polling behavior preserved

This change intentionally does not alter the rest of the transport model:

• Background tabs still poll at 25000ms
• Existing exponential error backoff is unchanged
• Failed polls do not contribute to RTT sampling
• retryNow() continues to affect retry timing only, not latency history
• No browser sniffing or UA detection was added; this uses feature detection only

Result

With this change:

• Chromium browsers get an immediate network-quality hint from effectiveType
• Firefox / Safari / iOS browsers can still reduce polling on slower conditions via measured RTT
• RTC polling remains conservative, capped, and compatible with the existing sync and awareness behavior

Use of AI Tools

Codex was used to strategize and plan and also for testing. Functionality is properly tested by human.

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Co-authored-by: the-hercules <thehercules@git.wordpress.org>
Co-authored-by: chrisdavidmiles <chrisdavidmiles@git.wordpress.org>

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