conversation: hide "scroll to end" when unnecessary

GitLab: #1251
Change-Id: I2a4d01560218daefb742af79565eb9b0da6c19a1

jami-android/app/src/main/java/cx/ring/fragments/ConversationFragment.kt

@@ -347,24 +347,29 @@ class ConversationFragment : BaseSupportFragment<ConversationPresenter, Conversa
                 val visibleLatestThreshold = 8
                 override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {}
                 override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
-                    val layoutManager = recyclerView.layoutManager as LinearLayoutManager?
-                    if (!loading && layoutManager!!.findFirstVisibleItemPosition() < visibleLoadThreshold) {
+                    val layoutManager = recyclerView.layoutManager as LinearLayoutManager? ?: return
+                    if (!loading
+                        && layoutManager.findFirstVisibleItemPosition() < visibleLoadThreshold
+                    ) {
                         loading = true
                         presenter.loadMore()
                     }
-                    if (layoutManager!!.itemCount - layoutManager.findLastVisibleItemPosition() > visibleLatestThreshold) {
+
+                    // Recyclerview is composed of items which are sometimes invisible (to preserve
+                    // the model and interaction relationship).
+                    // Because of bug #1251, we use findLastCompletelyVisibleItemPosition because
+                    // findLastVisibleItemPosition ignores invisible items (don't understand why).
+                    val lastVisibleItemPosition =
+                        layoutManager.findLastCompletelyVisibleItemPosition()
+                    if (layoutManager.itemCount - lastVisibleItemPosition > visibleLatestThreshold)
                         binding.fabLatest.show()
-                    } else {
-                        binding.fabLatest.hide()
-                    }
+                    else binding.fabLatest.hide()
                 }
             })
+
             val animator = binding.histList.itemAnimator as DefaultItemAnimator?
             animator?.supportsChangeAnimations = false
             binding.histList.adapter = mAdapter
-
-//            val toolbarLayout: AppBarLayout? = activity?.findViewById(R.id.toolbar_layout)
-//            toolbarLayout?.isLifted = true
         }
     }