test: add verifyStickyBar helper for sticky-element tests

[eran132]

Summary

Adds a single test helper, verifyStickyBar(page, opts), that captures a position: sticky element's background in both at-rest and scrolled states, and returns the metrics so callers write the exact assertions they want.

Returned shape:

• atRestBg, scrolledBg — computed background-color in each state
• conditionallyOpaque — convenience boolean: the bg differs between states (i.e. transparent at rest, opaque when stuck)
• stuckAtTop — the bar is still pinned within the top 100px after scrolling

Motivation

This directly prevents the bug pattern we hit twice during the #1528 (sticky inputs) review:

1. Original 'transparent bar lets content bleed through' — automated lint/tsc/unit all green; only a scrolled screenshot revealed the timeline timestamps showing through the sticky bar.
2. 'Always-opaque bar shows as a visible box on pages whose surrounding container color doesn't match' — the regression NoamGaash flagged; again invisible to all automated gates, only visible at rest on the gaps page.

Both could have been one line each:

const m = await verifyStickyBar(page, { selector: '[data-sticky-inputs]' })
expect(m.atRestBg).toBe('rgba(0, 0, 0, 0)')         // would have caught the box regression
expect(m.scrolledBg).not.toBe('rgba(0, 0, 0, 0)')   // would have caught the bleed-through
expect(m.stuckAtTop).toBe(true)                      // confirms it actually stuck

Implementation notes

Uses page.mouse.wheel, not window.scrollTo — this codebase has an inner scroll container, so window.scrollTo is a no-op (a lesson from #1528).

Test plan

• TypeScript / ESLint / Prettier clean
• Helper not exercised by an existing spec in this PR — the only sticky bar currently in the codebase arrives with feat: add sticky inputs section to gaps and timeline pages #1528, which isn't merged yet. Once it merges, applying it (e.g. inside tests/timeline.spec.ts) is one line. I'd rather ship the helper now so future sticky-bar work can pick it up, than block on PR ordering.

Closes nothing; this is pure infrastructure to make future visual reviews faster.

🤖 Generated with Claude Code

[github-actions]

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[Copilot]

Pull request overview

Adds a new Playwright test helper intended for future position: sticky UI tests by capturing a sticky bar’s computed background color in both at-rest and scrolled states, and returning simple metrics for assertions.

Changes:

• Introduces verifyStickyBar(page, opts) helper to read computed background-color before/after scrolling.
• Implements wheel-based scrolling to support inner scroll containers and returns conditionallyOpaque + stuckAtTop signals.

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