the component should use position:absolute;

[hzmsrv]

some situation it will keep the position at the center of the screen when the scroll bar at the middle and it is open at the top

Browsers Affected

• [X ] Chrome
• [ ] Firefox
• [ ] Safari 9
• [ ] Safari 8
• [ ] Safari 7
• [ ] Edge
• [ ] IE 11
• [ ] IE 10

[valdrinkoshi]

Duplicate of #128, see more details here https://github.com/PolymerElements/iron-dropdown/issues/128#issuecomment-377461543