Full size app iframes render with scroll bar

Issue Summary

It seems apps design to occupy the full page are rendered with an extra scroll bar. An example app has been created to demonstrate this as shown here. The extra scroll bar disappears after the window is resized.

Steps to Reproduce

1. Install the minimal demo app using the descriptor URL https://dx-general-page-app.glitch.me/connect.json. The source of this code is available at https://glitch.com/edit/#!/dx-general-page-app.
2. Change the window size so the height is about 800px.
3. Click on the "DX General Page" link in the "Apps" dropdown menu.
4. View the scrollbar(s) on the page.
5. Change the window size.
6. View the scrollbar(s) on the page.

Expected Results

At steps 4 and 6, the window has only one vertical scroll bar.

Actual Results

At step 4, two vertical scroll bars are visible. 

Workaround

None