[DatePicker] Dates flickering on fast hover in Chrome

[bamaboy]

Problem description

DatePicker's dates and ok button flickers, when you hover DatePicker just after it opened

Link to minimal working code that reproduces the issue

Visit http://www.material-ui.com/#/components/date-picker
Move your mouse over DatePicker right after it opened

Versions

• Material-UI: 0.18.1
• React: 15.4.0
• Browser: Google Chrome 58.0.3029.110 (64-bit)

[oliviertassinari]

Closing for #4787