[Select] multi label stays active when value is empty

[sakulstra]

Expected Behavior

When clearing a multiselect the label should get inactive again and the formcontrol should lose dirty state again.

Current Behavior

The label stays active.

Reproduction

Select an item in current multiselect docs and then deselect it again - the label will stay active.

I can fix it locally by adding

    this.checkDirty(nextProps);

to https://github.com/callemall/material-ui/blob/v1-beta/src/Input/Input.js#L377 but I don't understand why componentWillUpdate doesn't fire when items get unselected :/

[oliviertassinari]

The root of the issue is in #5186 where the focus, hover and selected state have the same UI.