MultiGrid misalignment when used with ColumnSizer

[laurence-hudson-mindfoundry]

I believe I've found a bug in MultiGrid which appears when it is used with ColumnSizer.

Specifically, when the column width is bigger than its minimum value the header can become misaligned with the data.

Interestingly this doesn't happen on first render, and only occurs after you start to scroll vertically. Once you start to scroll vertically a horizontal scroll bar will (incorrectly) appear, which if used breaks the alignment of the fixed header

There is an example of this behaviour here: https://next.plnkr.co/edit/v2N7VVkjOnJava3o?preview

It looks like this used to be an issue for MultiGrid, even when used without ColumnSize, but got fixed, without accounting for ColumnSizer.

Browser	chrome & edge (for sure, maybe others too)
OS	win7 (for sure, maybe others too)
React	15 & 16 (for sure, maybe others too)
React DOM	15 & 16 (for sure, maybe others too)
react-virtualized	9.20.1 (for sure, maybe others too)