(fix) preserve Study Rail height while moving

[Ammaar-Alam]

Summary

• preserve the Study Rail's rendered height when pointer or keyboard movement begins
• clamp the rail's position instead of expanding it as it approaches the top
• shrink only when a smaller viewport requires it and keep Home/reset anchor sizing unchanged
• cover the behavior with unit geometry and real Chrome pointer-drag regressions

Reproduction

Before the fix, moving the rail from top: 220px to top: 16px increased its unit-test height cap from 564px to 768px. The Chrome fixture likewise expanded the rendered rail from 756px to 764px during an upward drag.

Verification

• pnpm check
• 144 unit tests passed
• 60 Chrome tests passed
• lint, typecheck, build, and release audit passed

(PR written by Codex)

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Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: 5d2408dc94

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