Fix pages bottom sheet scroll

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/thumbnails/PagesThumbnailsSheet.kt

@@ -16,23 +16,25 @@ import org.koitharu.kotatsu.core.model.parcelable.ParcelableManga
 import org.koitharu.kotatsu.core.prefs.AppSettings
 import org.koitharu.kotatsu.core.ui.list.BoundsScrollListener
 import org.koitharu.kotatsu.core.ui.list.OnListItemClickListener
-import org.koitharu.kotatsu.core.ui.list.ScrollListenerInvalidationObserver
 import org.koitharu.kotatsu.core.ui.list.decor.SpacingItemDecoration
 import org.koitharu.kotatsu.core.ui.sheet.AdaptiveSheetBehavior
 import org.koitharu.kotatsu.core.ui.sheet.AdaptiveSheetCallback
 import org.koitharu.kotatsu.core.ui.sheet.BaseAdaptiveSheet
+import org.koitharu.kotatsu.core.util.RecyclerViewScrollCallback
 import org.koitharu.kotatsu.core.util.ext.observe
 import org.koitharu.kotatsu.core.util.ext.observeEvent
+import org.koitharu.kotatsu.core.util.ext.plus
 import org.koitharu.kotatsu.core.util.ext.scaleUpActivityOptionsOf
 import org.koitharu.kotatsu.core.util.ext.withArgs
 import org.koitharu.kotatsu.databinding.SheetPagesBinding
 import org.koitharu.kotatsu.list.ui.MangaListSpanResolver
+import org.koitharu.kotatsu.list.ui.model.ListModel
 import org.koitharu.kotatsu.parsers.model.Manga
 import org.koitharu.kotatsu.reader.ui.ReaderActivity
 import org.koitharu.kotatsu.reader.ui.ReaderState
 import org.koitharu.kotatsu.reader.ui.thumbnails.adapter.PageThumbnailAdapter
-import org.koitharu.kotatsu.reader.ui.thumbnails.adapter.TargetScrollObserver
 import javax.inject.Inject
+import kotlin.math.roundToInt
 
 @AndroidEntryPoint
 class PagesThumbnailsSheet :
@@ -79,14 +81,8 @@ class PagesThumbnailsSheet :
 			spanResolver?.setGridSize(settings.gridSize / 100f, this)
 			addOnScrollListener(ScrollListener().also { scrollListener = it })
 			(layoutManager as GridLayoutManager).spanSizeLookup = spanSizeLookup
-			thumbnailsAdapter?.registerAdapterDataObserver(
-				ScrollListenerInvalidationObserver(this, checkNotNull(scrollListener)),
-			)
-			thumbnailsAdapter?.registerAdapterDataObserver(TargetScrollObserver(this))
-		}
-		viewModel.thumbnails.observe(viewLifecycleOwner) {
-			thumbnailsAdapter?.setItems(it, listCommitCallback)
 		}
+		viewModel.thumbnails.observe(viewLifecycleOwner, ::onThumbnailsChanged)
 		viewModel.branch.observe(viewLifecycleOwner, ::updateTitle)
 		viewModel.onError.observeEvent(viewLifecycleOwner, SnackbarErrorObserver(binding.recyclerView, this))
 	}
@@ -124,6 +120,28 @@ class PagesThumbnailsSheet :
 		}
 	}
 
+	private fun onThumbnailsChanged(list: List<ListModel>) {
+		val adapter = thumbnailsAdapter ?: return
+		if (adapter.itemCount == 0) {
+			var position = list.indexOfFirst { it is PageThumbnail && it.isCurrent }
+			if (position > 0) {
+				val spanCount = spanResolver?.spanCount ?: 0
+				val offset = if (position > spanCount + 1) {
+					(resources.getDimensionPixelSize(R.dimen.manga_list_details_item_height) * 0.6).roundToInt()
+				} else {
+					position = 0
+					0
+				}
+				val scrollCallback = RecyclerViewScrollCallback(requireViewBinding().recyclerView, position, offset)
+				adapter.setItems(list, listCommitCallback + scrollCallback)
+			} else {
+				adapter.setItems(list, listCommitCallback)
+			}
+		} else {
+			adapter.setItems(list, listCommitCallback)
+		}
+	}
+
 	private inner class ScrollListener : BoundsScrollListener(3, 3) {
 
 		override fun onScrolledToStart(recyclerView: RecyclerView) {
@@ -133,6 +151,13 @@ class PagesThumbnailsSheet :
 		override fun onScrolledToEnd(recyclerView: RecyclerView) {
 			viewModel.loadNextChapter()
 		}
+
+		override fun onPostScrolled(recyclerView: RecyclerView, firstVisibleItemPosition: Int, visibleItemCount: Int) {
+			super.onPostScrolled(recyclerView, firstVisibleItemPosition, visibleItemCount)
+			if (firstVisibleItemPosition > offsetTop) {
+				viewModel.allowLoadAbove()
+			}
+		}
 	}
 
 	private inner class SpanSizeLookup : GridLayoutManager.SpanSizeLookup() {

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/thumbnails/PagesThumbnailsViewModel.kt

@@ -39,6 +39,7 @@ class PagesThumbnailsViewModel @Inject constructor(
 	private var loadingJob: Job? = null
 	private var loadingPrevJob: Job? = null
 	private var loadingNextJob: Job? = null
+	private var isLoadAboveAllowed = false
 
 	val thumbnails = MutableStateFlow<List<ListModel>>(emptyList())
 	val branch = MutableStateFlow<String?>(null)
@@ -51,8 +52,17 @@ class PagesThumbnailsViewModel @Inject constructor(
 		}
 	}
 
+	fun allowLoadAbove() {
+		if (!isLoadAboveAllowed) {
+			loadingJob = launchJob(Dispatchers.Default) {
+				isLoadAboveAllowed = true
+				updateList()
+			}
+		}
+	}
+
 	fun loadPrevChapter() {
-		if (loadingJob?.isActive == true || loadingPrevJob?.isActive == true) {
+		if (!isLoadAboveAllowed || loadingJob?.isActive == true || loadingPrevJob?.isActive == true) {
 			return
 		}
 		loadingPrevJob = loadPrevNextChapter(isNext = false)
@@ -74,7 +84,7 @@ class PagesThumbnailsViewModel @Inject constructor(
 	private suspend fun updateList() {
 		val snapshot = chaptersLoader.snapshot()
 		val mangaChapters = mangaDetails.tryGet().getOrNull()?.chapters.orEmpty()
-		val hasPrevChapter = snapshot.firstOrNull()?.chapterId != mangaChapters.firstOrNull()?.id
+		val hasPrevChapter = isLoadAboveAllowed && snapshot.firstOrNull()?.chapterId != mangaChapters.firstOrNull()?.id
 		val hasNextChapter = snapshot.lastOrNull()?.chapterId != mangaChapters.lastOrNull()?.id
 		val pages = buildList(snapshot.size + chaptersLoader.size + 2) {
 			if (hasPrevChapter) {