[Table] onVisibleCellsChange not called on scroll in React 16.x

[jdiprizio]

Bug report

• Package version(s): "@blueprintjs/core": "^1.35.0" "@blueprintjs/table": "^1.31.0"
• Browser and OS versions: Chrome 63.0.3239.108 Ubuntu 17.10

Steps to reproduce

1. Add handler with console.log for onVisibleCellsChange on a table with fixed height and lots of rows
2. Scroll table
3. Note that handler is called on mount but not on scroll

Actual behavior

onVisibleCellsChange is not called on scroll

Expected behavior

onVisibleCellsChange should be called on scroll

Fix

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.

[giladgray]

@jdiprizio thanks for the bug report, and double thanks for finding the offending code!! would happily accept a PR for this.