Date picker doesn't open properly for iOS < 9

I'm facing the issue that for iOS 8, 7 and below the date picker does not open properly. In more detail I think the class "picker--opened" is not set on .picker. I included the legacy.js.
The popup seems to be open - but is not visible - after clicking into the input-trigger. In that case the whole page is blocked. What do I need to do in order to make it working?
[ios-issue.mov.zip](https://github.com/amsul/pickadate.js/files/571773/ios-issue.mov.zip)

Cheers! :)