Document overscanIndicesGetter property

Author: clauderic

docs/Grid.md

@@ -22,6 +22,7 @@ A windowed grid of elements. `Grid` only renders cells necessary to fill itself
 | onSectionRendered | Function |  | Callback invoked with information about the section of the Grid that was just rendered. This callback is only invoked when visible rows have changed: `({ columnOverscanStartIndex: number, columnOverscanStopIndex: number, columnStartIndex: number, columnStopIndex: number, rowOverscanStartIndex: number, rowOverscanStopIndex: number, rowStartIndex: number, rowStopIndex: number }): void` |
 | onScroll | Function |  | Callback invoked whenever the scroll offset changes within the inner scrollable region: `({ clientHeight: number, clientWidth: number, scrollHeight: number, scrollLeft: number, scrollTop: number, scrollWidth: number }): void` |
 | overscanColumnCount | Number |  | Number of columns to render before/after the visible slice of the grid. This can help reduce flickering during scrolling on certain browsers/devices. |
+| overscanIndicesGetter | Function |  | Responsible for calculating the number of cells to overscan before and after a specified range [Learn more](#overscanIndicesGetter) |
 | overscanRowCount | Number |  | Number of rows to render above/below the visible slice of the grid. This can help reduce flickering during scrolling on certain browsers/devices. |
 | rowCount | Number | ✓ | Number of rows in grid. |
 | rowHeight | Number or Function | ✓ | Either a fixed row height (number) or a function that returns the height of a row given its index: `({ index: number }): number` |
@@ -147,6 +148,10 @@ function cellRangeRenderer ({
 }
 ```
 
+### overscanIndicesGetter
+This is an advanced property.
+This function is responsible for calculating the number of cells to overscan before and after a specified range. By default, React Virtualized optimizes the number of cells to overscan based on scroll direction. If you'd like to customize this behavior, you may want to fork the [`defaultOverscanIndicesGetter`](https://github.com/bvaughn/react-virtualized/blob/master/source/Grid/utils/defaultOverscanIndicesGetter.js) function.
+
 ### cellRenderer
 
 Responsible for rendering a single cell, given its row and column index.
@@ -173,15 +178,15 @@ function cellRenderer ({
   // Style is required since it specifies how the cell is to be sized and positioned,
   // and React Virtualized depends on this sizing/positioning for proper scrolling behavior.
   // By default, the grid component specifies, calculates, and initializes the following style properties:
-  //    height 
-  //    width 
+  //    height
+  //    width
   //    left
   //    top
   //    position
   // You can add additional class names or style properties as you would like.
   // Key is also required by React to more efficiently manage the array of cells.
   return (
-    <div 
+    <div
       key={key}
       style={style}
     >