immutableTop for prepend adapter method

Author: dhilt

src/modules/adapter.js

@@ -114,8 +114,8 @@ class Adapter {
     this.viewport.clipBottom();
   }
 
-  prepend(newItems) {
-    this.buffer.prepend(newItems);
+  prepend(newItems, options = {}) {
+    this.buffer.prepend(newItems, options.immutableTop);
     this.doAdjust();
     this.viewport.clipTop();
     this.viewport.clipBottom();

src/modules/buffer.js

@@ -24,9 +24,14 @@ export default function ScrollBuffer(elementRoutines, bufferSize, startIndex) {
       buffer.maxIndex = buffer.eof ? buffer.next - 1 : Math.max(buffer.next - 1, buffer.maxIndex);
     },
 
-    prepend(items) {
+    prepend(items, immutableTop) {
       items.reverse().forEach((item) => {
-        --buffer.first;
+        if (immutableTop) {
+          ++buffer.next;
+        }
+        else {
+          --buffer.first;
+        }
         buffer.insert('prepend', item);
       });
       buffer.minIndex = buffer.bof ? buffer.minIndex = buffer.first : Math.min(buffer.first, buffer.minIndex);