Fix pagination when the Calendar size is larger than the month size.

library/src/main/java/com/kizitonwose/calendarview/CalendarView.kt

@@ -6,7 +6,6 @@ import android.view.View.MeasureSpec.UNSPECIFIED
 import android.view.ViewGroup
 import androidx.annotation.Px
 import androidx.recyclerview.widget.DiffUtil
-import androidx.recyclerview.widget.PagerSnapHelper
 import androidx.recyclerview.widget.RecyclerView
 import com.kizitonwose.calendarview.model.*
 import com.kizitonwose.calendarview.ui.*
@@ -563,7 +562,7 @@ open class CalendarView : RecyclerView {
         }
     }
 
-    private val pagerSnapHelper = PagerSnapHelper()
+    private val pagerSnapHelper = CalenderPageSnapHelper()
 
     /**
      * Setup the CalendarView. You can call this any time to change the

library/src/main/java/com/kizitonwose/calendarview/ui/CalenderPageSnapHelper.kt

@@ -0,0 +1,50 @@
+package com.kizitonwose.calendarview.ui
+
+import android.view.View
+import androidx.recyclerview.widget.OrientationHelper
+import androidx.recyclerview.widget.PagerSnapHelper
+import androidx.recyclerview.widget.RecyclerView
+
+class CalenderPageSnapHelper : PagerSnapHelper() {
+
+    /**
+     * The default implementation of this method in [PagerSnapHelper.calculateDistanceToFinalSnap] uses the distance
+     * between the target view center vs RecyclerView center as final snap distance. This does not always give the
+     * desired result for calendar usage. For example in a vertical calendar when the RecyclerView is taller than
+     * the item view(e.g two or more visible months), we don't actually want the item view's center to be at the
+     * center of the RecyclerView when it snaps but instead we want the item view and RecyclerView top(in vertical)
+     * or left(in horizontal) to match at the end of the snap.
+     */
+    override fun calculateDistanceToFinalSnap(layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray {
+        return IntArray(2).apply {
+            this[0] = if (layoutManager.canScrollHorizontally())
+                distanceToStart(targetView, getHorizontalHelper(layoutManager)) else 0
+
+            this[1] = if (layoutManager.canScrollVertically())
+                distanceToStart(targetView, getVerticalHelper(layoutManager)) else 0
+        }
+    }
+
+    private fun distanceToStart(targetView: View, helper: OrientationHelper): Int {
+        val childStart = (helper.getDecoratedStart(targetView))
+        val containerStart = helper.startAfterPadding
+        return childStart - containerStart
+    }
+
+    private lateinit var verticalHelper: OrientationHelper
+    private lateinit var horizontalHelper: OrientationHelper
+
+    private fun getVerticalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper {
+        if (!::verticalHelper.isInitialized || verticalHelper.layoutManager != layoutManager) {
+            verticalHelper = OrientationHelper.createVerticalHelper(layoutManager)
+        }
+        return verticalHelper
+    }
+
+    private fun getHorizontalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper {
+        if (!::horizontalHelper.isInitialized || horizontalHelper.layoutManager != layoutManager) {
+            horizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager)
+        }
+        return horizontalHelper
+    }
+}