paper-menu-button Alternate Alignment shows an horizontal scroll bar

[phistuck]

Description

Using the paper-menu-button Alternate Alignment demo, if there is not enough real estate for the menu to show in its entirety and a vertical scroll bar shows up in the menu and (for Chrome - mousing down on one of the menu options would trigger) an additional horizontal scroll bar.

Expected outcome

No horizontal scroll bar.

Actual outcome

Horizontal scroll bar.

Live Demo

https://www.webcomponents.org/element/PolymerElements/paper-menu-button/demo/demo/index.html
Scroll down to "Alternate Alignment".

Steps to reproduce

1. Click on the menu in order to show it (make sure it has a vertical scroll bar. If it does not, make your window shorter until there is not enough room to show the top of the menu).
   (For Chrome, 2. Mouse down for a few seconds on one of the options.)

Since I know you do not have Windows (otherwise you would have seen the issue ;)), you can ask the Chromium team for access to BrowserStack, which provides you with remote Windows browsers.

Browsers Affected

• Chrome 58 on Windows (but only on mouse down)
• Chrome 58 on macOS (with overlay scroll bars) using BrowserStack
• Firefox 53 on Windows using BrowserStack
• Firefox 53 on macOS (with overlay scroll bars) using BrowserStack
• Safari 10 using BrowserStack
• Edge using BrowserStack
• IE 11 using BrowserStack
  Have not tested -
• IE 10
• Safari 9
• Safari 8
• Safari 7

Chrome 58 on Windows 7 -

Firefox 53 on Windows 10 -