Isolated iframe scrolling page

Repro steps:

Load on OSX.
Scroll the iframe to the bottom of the contents (but keep the page near the top).
Click the 'Add more iframe content' button to add more items to the iframe.
Attempt to scroll the iframe down to view the added items.

Observed:

Scrolling down immediately after adding content scrolls page instead of iframe.
Scrolling up just a bit will fix the vertical scroll of the iframe.

Expected:

After iframe content is added, iframe can be scrolled down.