Instant: stop stale background sync when embedded views are disposed (workaround + stress example)

[mutumbakato]

Update (10 Aug 2026): the underlying bug is now fixed upstream in Nutrient Android SDK 11.6.2. This PR no longer ships a workaround — it bumps the SDK pin and keeps the stress example as a regression harness. The original adapter workaround is preserved in the history (cd66772) for anyone pinned to an older SDK.

Problem

Closing a NutrientInstantView on Android did not stop that document's listen-for-server-changes long-poll. The InstantPdfDocument survived in the descriptor cache with its sync coordinator still listening, so every document ever viewed kept a background /sync connection open. After viewing enough distinct documents, these stale long-polls exhausted the per-host connection pool and the next document's auth/download queued behind them:

• Document loads stalled for 40–60 seconds on a healthy local Document Engine, no fault injection.
• Behind a reverse proxy with a read timeout, the stale polls surfaced as endless empty-body 502s for previously viewed documents.
• iOS was unaffected: it correctly stops sync on view teardown (verified side by side).

Reported in ZD#134164.

The fix

Nutrient Android SDK 11.6.2 (released 6 Aug 2026, AND-1957):

Fixes slow Instant document loads when frequently switching between documents. Disabling listening for server changes (including closing an embedded Instant view) now cancels the in-flight sync long-poll instead of leaving it holding a connection until the server releases it.

This is the proper fix — real-time listening stays enabled, and no adapter or periodic sync timer is needed.

What this PR contains

1. Bumps the pinned Nutrient Android SDK in android/config.gradle from 11.5.1 to 11.6.2.
2. Adds the "Instant Switch Stress" example — automates document-switch / close-reopen cycles against a Document Engine and reports per-cycle load latency. Retained as a regression harness now that the bug is fixed.
3. Removes the InstantSyncLifecycleAdapter workaround that the first commit introduced, along with its UI toggle and syncNow() helper.
4. Fixes example/pubspec.yaml — removes dependency_overrides pointing outside the repository (a monorepo sync artifact that broke pub get on clean checkouts).

Verification

12 distinct, never-before-downloaded documents opened in sequence; profile build on a clean API 36 emulator against a local Document Engine; no workaround active in either run.

	11.5.1	11.6.2
Median load	1392 ms	734 ms
Worst load	90014 ms (harness cap)	1386 ms
Timeouts	2 / 12	0 / 12
Wall clock for 12 cycles	~3 m 45 s	30 s

Re-verified after deleting the adapter: median 732 ms, worst 2115 ms (first-cycle cold start), 0 timeouts.

Not a document-specific artifact — fresh-doc-23 hit the 90 s cap on 11.5.1 and loaded in 1386 ms on 11.6.2.

For reference, the workaround measured in the first commit reached ~250 ms median with no stalls on 11.5.1; it is no longer required.

Note on the federated platform packages

nutrient_flutter_android hardcodes its own SDK default in android/build.gradle — 1.2.0 pins 11.5.1 and 2.0.0 pins 11.6.1, both below the fix. It is included as a Gradle subproject, so dependencyInsight shows 11.5.1 -> 11.6.2 resolved by conflict resolution and this build is correct. A matching bump is still needed in the platform-packages repo, otherwise an app resolving the federated package without the inline plugin lands below the fix.

Try it

cd example && flutter run
# open "Instant Switch Stress (repro #134164)", Run

The example defaults to a local Document Engine on :5001 with documents fresh-doc-01..30 and clean-doc-a uploaded (see the header of instant_switch_stress_example.dart for setup).

Apps that cannot take a plugin update yet can pick up the SDK fix directly by adding pspdfkit.version=11.6.2 to their app's android/local.properties.