[SelectField][DropDownMenu] Prevent label from resizing parents

[JessThrysoee]

When the first selection is made in SelectField with no previous selection, it affects the size of the parent elements. This can be seen in the Nullable, Floating label, and ErrorText examples on http://www.material-ui.com/#/components/select-field

Here are side-by-side screenshots of the difference. Both the size of the box and the position of the underline has changed:

This can be prevented by positioning the label absolutely instead of relatively.

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[oliviertassinari]

I confirm the issue and that the PR is addressing it. Thanks.
My only concern now is. Is this the right way to fix it? I'm gonna have to look at the alternatives. Any idea?

[JessThrysoee]

@oliviertassinari Did you come up with an alternative or any hints? Otherwise I'll take another look to see if we can avoid absolute positioning.

[oliviertassinari]

@JessThrysoee I haven't found the time needed yet. I just want to be sure the solution is a correct one. Positioning can be tricky and introduce other issue.

[JessThrysoee]

@oliviertassinari Good call, it turns out the label simply needs a height.

[oliviertassinari]

I can't control the side effect of that change without the visual regression test suite we have on the next branch. That's fixing the issue, hopefully, without introducing a regression.