fix: zoom should be wrt the current scrubbers

[tibisabau]

Summary

Fixed zoom functionality to zoom relative to the current ruler position instead of from the left edge of the timeline. This provides a more intuitive zoom experience where the playhead stays centered during zoom operations.

Changes

• Modified handleZoomIn, handleZoomOut, and handleZoomReset in useTimeline.ts to accept an optional centerPx parameter representing the zoom center point
• Updated zoom calculations to apply an offset that keeps the zoom center point visually stable during zoom operations
• Updated all zoom function calls in home.tsx to pass the current rulerPositionPx as the zoom center
• Updated LLM handler zoom functions to support the new optional centerPx parameter for consistency

The zoom now uses the formula: newLeft = scrubber.left * zoomRatio + centerPx * (1 - zoomRatio), which ensures the timeline zooms toward/away from the ruler position rather than always from the left edge.

Testing

• Verified ESLint passes with no warnings on modified files
• Confirmed TypeScript type checking shows no errors in modified files
• Tested zoom behavior matches expected behavior: zooming in/out now keeps the ruler position stable in the viewport

Screenshots / Recordings (if UI)

N/A

Related Issues

Closes #104

Breaking Changes

None. The zoom functions now accept an optional parameter with a default value of 0, maintaining backward compatibility.

Deployment Notes

None required.

The code in this pull request was generated by GitHub Copilot with the Claude Sonnet 4.5 model.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the user experience of the timeline's zoom feature by making it relative to the current playhead position. Previously, zooming always occurred from the left edge, which could be disorienting. Now, the timeline zooms in and out around the user's current focus, providing a more natural and intuitive interaction. This change improves usability without introducing any breaking changes.

Highlights

• Intuitive Zoom Functionality: Implemented zoom functionality that centers on the current ruler (playhead) position, providing a more natural and intuitive user experience instead of zooming from the timeline's left edge.
• Updated Zoom Hooks: Modified handleZoomIn, handleZoomOut, and handleZoomReset in useTimeline.ts to accept an optional centerPx parameter, allowing zoom operations to pivot around a specified point.
• Adjusted Zoom Calculations: Adjusted the internal zoom calculations to apply an offset, ensuring the visual stability of the zoom center during scaling operations.
• UI Integration: Updated all zoom function calls in home.tsx (for both wheel events and UI buttons) to pass the current rulerPositionPx as the centerPx.
• LLM Handler Consistency: Extended LLM handler zoom functions in llm-handler.ts to support the new optional centerPx parameter, maintaining API consistency.

Changelog

• app/hooks/useTimeline.ts
  • Updated handleZoomIn, handleZoomOut, and handleZoomReset to accept an optional centerPx argument, defaulting to 0.
  • Introduced an offset calculation using centerPx * (1 - zoomRatio) to maintain the visual center during zoom operations.
  • Modified the left property calculation for scrubbers to scrubber.left * zoomRatio + offset.
• app/routes/home.tsx
  • Modified calls to handleZoomIn, handleZoomOut, and handleZoomReset to pass the rulerPositionPx as the centerPx argument.
  • Added rulerPositionPx to the dependency array of the useEffect hook managing the wheel event listener.
• app/utils/llm-handler.ts
  • Updated the signatures of llmZoomIn, llmZoomOut, and llmZoomReset to accept an optional centerPx parameter.
  • Passed the centerPx parameter to the corresponding handleZoom functions.

Activity

• No specific activity (comments, reviews, or progress updates) has been recorded for this pull request since its creation.

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[gemini-code-assist]

Code Review

This pull request correctly implements zooming relative to the ruler position, which provides a more intuitive user experience. The changes in useTimeline.ts, home.tsx, and llm-handler.ts are consistent and well-implemented. I've identified an opportunity to refactor the zoom handling logic in useTimeline.ts to reduce code duplication, which would improve maintainability.