Visible area not scrollable in the overlay containers on Reddit

[dceban]

Steps to reproduce:

1. Go to Reddit (new layout).
2. Open a post in an overlay container (directly from the timeline).
3. Initiate the "Screenshot" tool.
4. Try to scroll using the mouse-wheel or the keyboard buttons.

Expected results: The overlay container should be scrollable.
Actual results: The user is unable to scroll the visible area. Also, when using the screenshot tool to capture a div that is higher than the visible area, the buttons (X, Copy, Download) are not visible (but are actually displayed at the bottom of the div, where the user is unable scroll).

Short demo link: https://youtu.be/RJULe_OF9ow