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backlog/tasks/task-11 - Break-up-the-applyInvisibleSubtitleLayoutFromMpvMetrics-mega-function.md

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+---
+id: TASK-11
+title: Break up the applyInvisibleSubtitleLayoutFromMpvMetrics mega function
+status: To Do
+assignee: []
+created_date: '2026-02-11 08:21'
+labels:
+  - refactor
+  - renderer
+  - complexity
+milestone: Codebase Clarity & Composability
+dependencies: []
+references:
+  - src/renderer/renderer.ts
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+In renderer.ts (around lines 865-1075), `applyInvisibleSubtitleLayoutFromMpvMetrics` is a 211-line function with up to 5 levels of nesting. It handles OSD scaling calculations, platform-specific font compensation (macOS vs Linux), DPR calculations, ASS alignment tag interpretation (\an tags), baseline compensation, line-height fixes, font property application, and transform origin — all interleaved.
+
+Extract into focused helpers:
+- `calculateOsdScale(metrics, renderAreaHeight)` — pure scaling math
+- `calculateSubtitlePosition(metrics, scale, alignment)` — ASS \an tag interpretation + positioning
+- `applyPlatformFontCompensation(style, platform)` — macOS kerning/size adjustments
+- `applySubtitleStyle(element, computedStyle)` — DOM style application
+
+This can be done independently of or as part of TASK-6 (renderer split).
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 No single function exceeds ~50 lines in the positioning logic
+- [ ] #2 Helper functions are pure where possible (take inputs, return outputs)
+- [ ] #3 Platform-specific branches isolated into dedicated helpers
+- [ ] #4 Invisible overlay positioning still works correctly on Linux and macOS
+<!-- AC:END -->