Add pixel for max tab reuse distance

[catalinradoiu]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1211724162604201/task/1214069186174646
Tech Design URL (if applicable):
API Proposals URL(s) (if applicable): None

Description

Adds a pixel that reports, once per foreground session, the furthest a user went back in the tab activation order (the tab reuse distance), bucketed, alongside a bucketed count of currently open tabs. This distance is the condition that decides whether a given tab would still be retained if the fragment retention limit (MAX_ACTIVE_TABS) were lowered, so the pixel sizes the UX cost of doing that before the limit is actually changed.

The pixel is gated behind the tabManager.tabMaxReuseDistancePixel sub-feature, off by default, with the parent tabManager toggle checked explicitly at the call site.

Steps to test this PR

Feature 1: Tab reuse distance pixel fires once per foreground session

• Enable the tabManager.tabMaxReuseDistancePixel sub-feature (and ensure the parent tabManager feature is enabled)
• Open several tabs and switch between them so some tabs are reused further back in the activation order
• Background and foreground the app
• Confirm the pixel fires with bucketed reuse-distance and open-tab-count values, and only once per foreground session

Feature 2: Pixel stays off when the sub-feature is disabled

• Leave tabManager.tabMaxReuseDistancePixel disabled (default)
• Repeat the tab-switching steps above
• Confirm no pixel fires

UI changes

Before	After
No UI changes	No UI changes

---

Note

Low Risk
Analytics-only path behind feature flags; tab pager wiring adds hooks on activation/pause with no user-facing behavior changes.

Overview
Adds tab_max_reuse_distance telemetry to measure how far users jump back in tab activation order before revisiting a tab—aligned with the same “distinct other tabs since last active” notion used for fragment retention under MAX_ACTIVE_TABS.

A new activity-scoped TabReuseDistanceReporter tracks activations (via onItemPlaced on the tab pager adapter), tab closes, and open-tab count; on BrowserActivity.onPause it fires one bucketed pixel per foreground stretch (distance_bucket, tab_count_bucket) when the user actually returned to an earlier tab. Reporting is gated by tabManager and the new tabMaxReuseDistancePixel sub-toggle (default off). Pixel schema, AppPixelName, and unit tests cover bucketing, session reset on pause, removals, and toggle behavior.

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[catalinradoiu]

• Add pixel for max tab reuse distance #9755 👈 (View in Graphite)
• develop

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[github-actions]

Privacy Review task: https://app.asana.com/0/69071770703008/1218312506872025

[0nko]

Things worked as expected when testing, but I did notice that opening any activity constitutes an "end of session" and the pixel is sent. There's an implication to this because the max distance counter is reset to 0, so if you keep moving between screens like the settings or the tab switcher, you're technically in the same "session" but the reported data is artificially lower.

However, I guess it really comes down to what a "session" means and if we accept this behavior then I guess it's fine, so I'll approve. Maybe we should update the pixel definition description to capture that, though.