TXT chapter mode: progress bar shows book progress instead of chapter progress

[lllyys]

Bug

Severity: Medium

Repro

1. Open a TXT file that has detected chapters (WI-6 chapter-based mode)
2. Navigate to any chapter
3. Observe the ReadingProgressBar scrubber and progress percentage

Expected

Progress bar reflects position within the current chapter (0-100%).

Actual

Progress bar shows position relative to the whole book.

Root cause

currentOffsetUTF16 in TXTReaderViewModel is dual-purpose — set to a global offset by navigateToChapter() but overwritten with local (chapter-relative) offsets by the bridge's scroll callbacks. The onChange handler in TXTReaderContainerView that feeds chapterScrollFraction can't reliably distinguish them.

Attempted fixes

• Normalize offset by subtracting chapter.globalStartUTF16 when offset >= globalStart
• Still unreliable due to dual-purpose currentOffsetUTF16

Proper fix needed

Separate currentOffsetUTF16 (global, for persistence) from a chapter-local scroll fraction reported directly by the bridge.

Files

• vreader/Views/Reader/TXTReaderContainerView.swift — progress bar wiring
• vreader/ViewModels/TXTReaderViewModel.swift — currentOffsetUTF16, navigateToChapter()
• vreader/Views/Reader/TXTTextViewBridgeCoordinator.swift — scroll callbacks