Use 'absolute' positioning for menu instead of 'fixed'

[cdevienne]

The 'fixed' approach has a major downside:

The 'fixed' referential may not be the document when a parent node has CSS transforms. But getBoundedClientRect() remains relative to the document no matter what. Calculating a fixed position from it is difficult.

Using 'absolute' positioning makes things a lot easier. It is how bootstrap dropdown menu works for example.

[mihaibogdan10]

Also, on pages that have enough content to have a scroll, position: fixed fails horribly.

[mihaibogdan10]

@cdevienne There's no need for that maxWidth hack now, having position relative on the parent makes width: 100% work just fine 😄

[CMTegner]

Just a heads-up (in case you're waiting for this PR to land): You can fully customize the dropdown menu by using props.menuStyle and props.renderMenu.

We'll probably be removing most of the default menu styles for 2.0, possibly only keeping the absolute coordinates which are passed to props.renderMenu (although you could calculate this yourself if we added support for rendering custom components instead of just fragments for the menu).