Scroll when container contracted to hide current scroll

[ReinAkane]

Fixing issue #780: scrolling then reducing list size could cause all items to disappear in Firefox. See issue for a GIF.

The root cause is that Firefox does not fire a scroll event when a scrolled element's size decreases. This causes the Grid's scrollTop and scrollLeft values to be out of sync with the actual scroll location of the element.

This PR makes the Grid check for item count changes to determine if it needs to scroll the grid back into view.

[bvaughn]

Haven't tested this. I trust you. 😄

[bvaughn]

This feature has been released in version 9.10.0. Thank you for contributing!

[ReinAkane]

@mcordova47 I'm not going to have time to test for a while, sadly. If you want to pull it down and test yourself, I added unit tests to ensure that the bug this PR fixed continues to be fixed.

[bvaughn]

I think this introduced the regression described in #816 and bvaughn/react-virtualized-select/issues/75

Happy to review a fix if you'd like to put up a PR @mcordova47

[mcordova47]

@bvaughn @ReinAkane I looked a little further into it, and the problem I am facing is a little different than the problem described in #816. It seems to be a problem with the MultiGrid component that was exposed by this commit. MultiGrid has no default value for scrollToRow, and it sets its bottom right grid's scrollToRow to props.scrollToRow - props.fixedRowCount, causing it to be NaN when it should default to -1.

Adding defaults of -1 to scrollToRow and scrollToColumn in MultiGrid seems to fix my issue. I created a pull request (#820), if you'd like to review that. Some tests for other components were failing before my fix, but all tests for MultiGrid pass. Added two tests that were failing before the fix.