Bug Report

• Package version(s): 5.82.4
• Browser and OS versions: macOS 10.13.4, Chrome 65

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix? Yes

Requested priority: Normal

Products/sites affected: onedrive.live.com

Describe the issue:

A bug was introduced in #4055 which prevents a virtualized List rendered inside a ScrollablePane from rendering pages as they're scrolled into view.

Actual behavior:

Render a List or DetailsList inside a ScrollablePane, e.g.

<ScrollablePane>
  <DetailsList
    items={ items }
    columns={ columns }
    setKey='set'
    selection={ this._selection }
  />
</ScrollablePane>

Observe that, when there is a large number of items, pages below the fold are not rendered as they are scrolled into view. e.g. run npm start and visit http://localhost:4322/#/examples/scrollablepane:

Expected behavior:

Pages in the list should be rendered as they are scrolled into view.