Fix: flaky revisions pagination e2e cross-page diff assertion

[jorgefilipecosta]

Fixes #80154

Fixes the flaky "should paginate, navigate pages, and diff across page boundaries" e2e test.

After pressing Home the test read the two diff marks with one-shot textContent() calls, racing the canvas re-renders. Right after the keypress the canvas can still show the previously selected revision's diff, so the first read returns the stale added mark ("105"). While the adjacent page loads, the previous revision is unknown and the paragraph is briefly rendered as wholly added, with no removed mark in the DOM — so the second read auto-waits and lands on the settled cross-page diff ("5"). That produces Expected: 1, Received: 100 (105 − 5, always exactly the page size), the signature of every failure recorded in #80154.

The reads and the assertion are now wrapped in a polling expect( async () => { ... } ).toPass() block, so both marks are sampled together and retried until they show a consistent adjacent pair — a straddled sample fails the attempt and is retried instead of failing the test.

Testing Instructions

1. Run npm run test:e2e -- test/e2e/specs/editor/various/revisions.spec.js --grep "paginate" and verify it passes, e.g. with --repeat-each=10.
2. To reproduce the flake on trunk, widen the two race windows like a busy CI runner by applying the following patch, then run the test a few times without this PR's assertion change — it fails with Expected: 1, Received: 100. With this PR's change the same widened runs pass.

diff --git a/test/e2e/specs/editor/various/revisions.spec.js b/test/e2e/specs/editor/various/revisions.spec.js
--- a/test/e2e/specs/editor/various/revisions.spec.js
+++ b/test/e2e/specs/editor/various/revisions.spec.js
@@ -596,6 +596,20 @@
 		const slider = page.getByRole( 'slider', { name: 'Revision' } );
 		await expect( slider ).toHaveAttribute( 'max', '99' );
 
+		// Widen the race windows: delay the adjacent-page fetch and slow the
+		// canvas re-render, like a loaded CI runner.
+		await page.route(
+			( url ) =>
+				url.pathname.includes( '/revisions' ) &&
+				url.searchParams.get( 'page' ) === '2',
+			async ( route ) => {
+				await new Promise( ( r ) => setTimeout( r, 1500 ) );
+				await route.continue();
+			}
+		);
+		const cdp = await page.context().newCDPSession( page );
+		await cdp.send( 'Emulation.setCPUThrottlingRate', { rate: 6 } );
+
 		// Slide to the leftmost (oldest revision on page 1). Computing the
 		// previous-revision diff requires fetching the adjacent page.
 		await slider.focus();

The change was developed with the assistance of AI tooling.

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Co-authored-by: jorgefilipecosta <jorgefilipecosta@git.wordpress.org>

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Size Change: 0 B

Total Size: 7.81 MB

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