select value on focus

Summary: select the value of any input element once the field gets focus.

Reviewed By: emilsjolander

Differential Revision: D7009723

fbshipit-source-id: e1bc0f4b219958d8bdb79ef1a583cad86157b050

website/src/components/Playground/EditValue.js

@@ -36,6 +36,7 @@ export default (props: Props<*>) => {
         {...props}
         onChange={e => props.onChange(props.property, e.target.value)}
         placeholder="undefined"
+        onFocus={e => e.target.select()}
         value={Number.isNaN(props.value) ? '' : props.value}
       />
     );