feat: bind overlay state to secondary subtitle mpv visibility

docs/plans/2026-02-26-secondary-subtitles-main-overlay.md

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+# Secondary Subtitles Main Overlay Implementation Plan
+
+> **For Claude:** REQUIRED SUB-SKILL: Use superpowers:executing-plans to implement this plan task-by-task.
+
+**Goal:** Ensure secondary subtitles render in the unified main overlay window and remove stale secondary-window/layer paths.
+
+**Architecture:** Keep secondary subtitle DOM in the shared renderer tree, rely on mode classes (`secondary-sub-hidden|visible|hover`) for visibility, and remove obsolete legacy overlay-layer assumptions. Preserve modal behavior and existing subtitle rendering flow.
+
+**Tech Stack:** TypeScript, Electron renderer CSS/DOM, Bun test runner.
+
+---
+
+### Task 1: Add Regression Tests For Main Overlay Secondary Rendering
+
+**Files:**
+- Modify: `src/renderer/subtitle-render.test.ts`
+- Modify: `src/renderer/error-recovery.test.ts`
+
+**Step 1: Write failing tests**
+- Assert stylesheet no longer hides secondary subtitles in `layer-visible`.
+- Assert renderer platform resolution ignores legacy `secondary` overlay layer.
+
+**Step 2: Run tests to verify failures**
+
+Run: `bun test src/renderer/subtitle-render.test.ts src/renderer/error-recovery.test.ts`
+Expected: FAIL on secondary subtitle hide rule + legacy secondary layer handling.
+
+### Task 2: Remove Secondary-Window CSS/Routing Assumptions
+
+**Files:**
+- Modify: `src/renderer/style.css`
+- Modify: `src/renderer/utils/platform.ts`
+- Modify: `src/renderer/error-recovery.ts`
+- Modify: `src/types.ts`
+
+**Step 1: Implement minimal changes**
+- Remove legacy forced hide on `#secondarySubContainer`.
+- Remove obsolete layer-specific secondary-subtitle CSS blocks.
+- Drop legacy `secondary` overlay-layer parsing path from renderer platform resolver.
+- Narrow related overlay layer type unions.
+
+**Step 2: Run targeted tests**
+
+Run: `bun test src/renderer/subtitle-render.test.ts src/renderer/error-recovery.test.ts`
+Expected: PASS.
+
+### Task 3: Validate Wider Related Surface
+
+**Files:**
+- No additional code changes required.
+
+**Step 1: Run broader related tests**
+
+Run: `bun test src/renderer/subtitle-render.test.ts src/renderer/error-recovery.test.ts src/main/runtime/overlay-window-runtime-handlers.test.ts src/main/runtime/overlay-window-factory.test.ts src/core/services/overlay-manager.test.ts`
+Expected: Renderer tests pass; report any unrelated pre-existing failures.