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

id, title, status, assignee, created_date, updated_date, labels, milestone, dependencies, references, priority

id	title	status	assignee	created_date	updated_date	labels	milestone	dependencies	references	priority
TASK-11	Break up the applyInvisibleSubtitleLayoutFromMpvMetrics mega function	To Do		2026-02-11 08:21	2026-02-15 07:00	refactor renderer complexity	Codebase Clarity & Composability	TASK-27.5	src/renderer/renderer.ts	medium

Description

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).

Acceptance Criteria

• #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

Implementation Notes

Reparented as a dependency of TASK-27.5: the mega-function lives in positioning.ts (513 LOC), which is the exact file TASK-27.5 targets for splitting. Decomposing this function is a natural part of that file split. Should be executed together with TASK-27.5.