fix(reader): Preload adjacent chapters when current chapter has a single page#642
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…gle page When a chapter consists of a single page, the reader never scrolls onto a ChapterTransition page, so the adjacent-chapter preload that the transition page normally triggers never runs. This left the reader unable to swipe backwards out of a 1-page chapter (issue null2264#617). Track a prevTransition on PagerViewerAdapter (mirroring nextTransition). Move the single-page adjacent preload out of the isIdle-only path into setChaptersDoubleShift so it runs in both code paths, and preload the previous chapter in addition to the next one either direction can swipe out of the chapter.
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Closes #617
Summary
When a chapter consists of exactly one page, the reader never scrolls onto a
ChapterTransitionpage, so the adjacent-chapter preload that the transition page normally triggers never runs. Because the lone page settles at idle with the same first/last index, swiping backwards out of the 1-page chapter cannot reach a loaded previous chapter, leaving the reader stuck (the forward direction was already handled, but only for the next chapter).Changes
PagerViewerAdapter.kt: Track aprevTransition(ChapterTransition.Prev) mirroring the existingnextTransition, so the previous-adjacent chapter is reachable for inspection.PagerViewer.kt: Move the single-page adjacent-chapter preload out of theisIdle-only path intosetChaptersDoubleShiftso it runs regardless of which code path sets the chapters, and preload the previous chapter in addition to the next one.This lets the user swipe out of a 1-page chapter in either direction, since the preceding chapter is preloaded and the "Load Previous Chapter" transition can proceed.
Notes
1040eccd7e "Fix chapter transition setting for one page chapters"only preloadednextTransition?.to, and only when chapters arrived while the pager was non-idle. This regressed for the backward direction and for the idle path.requestPreloadChapter->ReaderViewModel.preload->Event.ReloadViewerChapters), so once the previous chapter loads the viewer is updated as usual.Testing
Manual repro from the issue: open a manga, ensure the previous chapter has exactly 1 page, read the current chapter, swipe back into the 1-page chapter, then swipe back again. The reader should now trigger the "Load Previous Chapter" transition instead of getting stuck. Build/CI verification via
./gradlew assembleStandardRelease+ unit tests.