[Phase 1] Scroll Depth Plugin

[prosdev]

Description

Implement scroll depth tracking plugin with industry-standard capabilities.

Tasks

• Research implementation (thresholds, scroll calc, throttling)
• Design API (thresholds, throttle, viewport height)
• Implement scroll percentage calculation
• Add throttled scroll listener
• Support multiple thresholds
• Emit trigger:scrollDepth events
• Write comprehensive tests (>80% coverage)

Acceptance Criteria

• Tracks scroll percentage accurately
• Throttling works (100ms default)
• Multiple thresholds supported
• Events emitted with correct data
• API: getMaxPercent(), getCurrentPercent(), reset()
• Handles dynamic content height changes
• No memory leaks

• 80% test coverage

Reference

• specs/phase-1-display-conditions/plan.md (Plugin 2)
• specs/phase-1-display-conditions/tasks.md (Tasks 3-5)