[HxGrid] ProgressInidicator incorrectly positioned when using InfiniteScroll (Virtualize)

[hakenr]

Repro:

1. /components/HxGrid#InfiniteScroll
2. Scroll a few rows down.
3. Hit the Refresh button.

The progress indicator is not vertically centered over the grid but "moves with the rows." When you scroll further down and then hit Refresh, the progress indicator is not visible at all.

We need the progress indicator to maintain its centered position (both horizontally and vertically).

Hope this won't require adding the extra div back into the structure.

cc @crdo

[hakenr]

The original progress indicator was replaced with a new one #842.