[Table] Call onVisibleCellsChange on scroll (in React 16.x)

[mrshll]

Fixes #1974

Changes proposed in this pull request:

From: #1974

The issues lies in updateViewportRect method in table.tsx.

Code below:

this.setState({ viewportRect: nextViewportRect });
const { viewportRect } = this.state;

const didViewportChange =
            (viewportRect != null && !viewportRect.equals(nextViewportRect)) ||
            (viewportRect == null && nextViewportRect != null);

is incorrect because viewportRect is destructured after the setState. React recent updates have changed the way setState is executed and state may be set immediately, thus causing didViewportChange to be false and the onVisibleCellsChange to not be invoked. Simply swapping the first two lines fixes the issue.

Reviewers should focus on:

didViewportChange working properly. The table performance perhaps improving via: #1664

[palantirtech]

Thanks for your interest in palantir/blueprint, @mmoutenot! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[themadcreator]

There are probably other subtle issues that we'll find with the semantic changes to .setState but this is good fix.