test: pin item height in date-picker overlay-content fixture (#11676) (CP: 25.1)

[vaadin-bot]

This PR cherry-picks changes from the original PR #11676 to branch 25.1.

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Original PR description

The customizeFixture helper in overlay-content.test.js set the month scroller height as 270 * monthScrollerItems px, hardcoding the theme-specific Lumo item height. The actual --vaadin-infinite-scroller-item-height is theme-dependent (264 px in the current base styles, different in upcoming changes), so revealDate position math drifted and two assertions had to fall back on closeTo(…, 0.1) plus FIXME comments.

Pin the CSS variable to 270px on the month scroller before it activates (i.e. before _createPool reads and caches itemHeight). With a clean integer item height, percentage-based buffer offsets resolve to whole pixels and the visible-area math is exact:

• monthScrollerItems: 2, no offset → visibleItems = 2 → expected position + 1
• monthScrollerItems: 3, 10% offset → bufferOffset = 81 px exact → visibleItems = 2.4 → expected position + 0.6

Drop the FIXMEs and tighten the two assertions from closeTo(…, 0.1) to equal(…) so any future drift fails loudly instead of being masked.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[vaadin-bot]

This ticket/PR has been released with Vaadin 25.1.7.