feat: add side-by-side comparison preview with draggable slider (#896)

[Pranav-IIITM]

Description

Added a side-by-side comparison preview with a draggable slider divider, letting users visually compare the original video against the reframed output without exporting.

New component: ComparisonPreview.tsx

• Left side: original video (object-contain, no overlay)
• Right side: same video with crop/framing overlay applied (reuses barTop/barBottom/barLeft/barRight logic from VideoPreview)
• Draggable vertical divider — white line with circular handle — drag left/right to reveal either side
• Both videos synced via timeupdate event (same currentTime, play/pause, playback rate)
• Touch + mouse events supported for full mobile compatibility
• Overlays are pointer-events-none, drag handle is pointer-events-auto
• No new dependencies — pure React hooks + Tailwind only

Updated: VideoPreview.tsx

• Added showComparison state
• Added "Compare" toggle button on the video preview
• Renders ComparisonPreview below main preview when toggled on

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Related Issue

Closes #896

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Type of Contribution

• Bug fix
• New feature
• Documentation update
• GSSoC contribution

Participant Info

• GitHub username: @Pranav-IIITM
• Contribution level: Intermediate

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Screen Recording

feat-side-by-side-video-preview.1.mp4

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Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

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[github-actions]

👋 Thanks for your PR, @Pranav-IIITM!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

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Quick checklist

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• Linked the issue this PR closes (e.g. Closes #123)
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Project	Deployment	Actions	Updated (UTC)
reframe	Ready	Preview, Comment	May 23, 2026 10:51am