fix(viewer): actively cancel superseded PDF.js work

[seonghobae]

Objective

Advance the remaining resource-cancellation acceptance in issue #322 without moving parent #323's exact reviewed/verified identity. A superseded viewer operation must not only suppress stale DOM publication; where PDF.js exposes cancellation/destruction APIs, it must actively cancel the current RenderTask and destroy an in-flight PDFDocumentLoadingTask.

Fresh exact stack state — 2026-08-10

• exact child head: 29438e42327a1bd5b888422fbfde713f7fca9b5a;
• exact parent fix(viewer): suppress stale PDF render publication #323 head: 8914580339c644f2cfdfef6a1634dfd5cff53693;
• parent fix(viewer): suppress stale PDF render publication #323 remains directly based on protected main 55d7ae8647208e301f282350f076eeddaba61d11;
• child is intentionally stacked on fix(viewer): suppress stale PDF render publication #323 because both slices own viewer.js and this run did not move the parent exact identity;
• exact-head CI 31406253421: success;
• exact-head fuzz 31406253440: success;
• CI checked out the exact child SHA, ran 496 Maven tests with 0 failures, 0 errors and 0 skips, passed all JaCoCo coverage checks and public Javadocs, passed buyer-readiness script tests, and passed synthetic parent-merge compatibility;
• no formal reviews or unresolved review threads at the latest direct refetch.

Because this stacked PR targets the parent branch rather than main, no main-targeted Security Scan/SAST result is synthesized as passing evidence. After #323 integrates and this child is rebuilt/reconciled onto the new exact protected main, all base-sensitive security/review evidence must be regenerated before final merge.

Test-first / RCA evidence

RED

Test-only head 5e7cbd2c6a868332ea8c1da371ce73cc46253d5d added a real Node runtime regression around PDF.js resources before production changes. Exact-head CI 31405521868 failed Buyer-readiness scripts exactly at the first intended boundary: aborting a viewer operation while the first-page RenderTask.promise was held pending produced cancelCalls = 0, proving parent #323 suppressed stale publication but did not actively cancel the renderer.

First GREEN attempt exposed a harness race

Production head 36bb461ed7d02febea446d3a1bce3c2fff163b99 added active cancellation/destruction, but exact-head script CI still reported cancelCalls = 0. RCA showed the regression used a fixed count of resolved microtasks before aborting; that did not prove page.render() had actually started in the Node VM. The failure was therefore timing-sensitive test synchronization, not proof that the new cancel listener was absent. The test was corrected to expose explicit renderStarted / loadingStarted synchronization boundaries instead of sleeping/yielding an assumed number of microtasks.

GREEN

Current renderPdfInline(...) now:

• retains the current PDF.js RenderTask and calls cancel() exactly once when the operation aborts;
• destroys a still-loading PDFDocumentLoadingTask on abort when PDF.js exposes destroy();
• guards cancellation with a one-shot state flag so abort observation cannot double-cancel resources;
• treats cancellation/destruction as best-effort resource cleanup while existing abort checks remain authoritative against stale publication;
• suppresses cancellation errors only when the caller's abort signal is actually set and continues to surface non-cancellation failures;
• removes the abort listener in finally and still destroys a resolved PDFDocumentProxy.

The synchronized Node regression proves both active render-task cancellation and loading-task destruction. The unchanged exact head is GREEN across buyer-readiness runtime scripts, full Maven verify/coverage/Javadocs, synthetic parent-merge compatibility and fuzz.

Scope / remaining issue contract

This child does not duplicate #323 stale-publication suppression, issue #317 identity/bootstrap work, #264 demo.js/dom-utils.js, signed-token authority, terminal focus ownership, or rapid three-generation/error-state coverage. Signed-token mode still invokes renderPdfInline without a generation signal; parity remains open under issue #322.

Do not close issue #322 when this child eventually integrates. Remaining acceptance includes rapid multi-generation/error-state coverage, signed-token lifecycle parity, terminal status/focus ownership, and proof that cancellation cannot surface an unhandled rejection in the supported PDF.js runtime.

Dependency gate

Keep this PR Draft while parent #323 is open. Merging the child into the parent branch would move #323's exact reviewed/verified identity and collapse two independently reviewable semantics. After #323 reaches protected main, rebuild/reconcile this unique delta onto the new exact protected base, regenerate all main-targeted CI/security/review evidence, and only then move the child toward final merge.

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