Scrolling a browser extension's popup window scrolls the main page

Find an extension with a popup window that scrolls (I tried with the [Cross-Origin Storage extension](https://addons.mozilla.org/en-US/firefox/addon/cross-origin-storage/)). Open the popup window and try scrolling. Observe how the main page scrolls instead of the popup window when you scroll with the mouse. (You can scroll by focusing the popup window and scrolling with the arrow keys.)

Reproduce the issue with the Cross-Origin Storage extension linked in the report by opening its popup and scrolling with the mouse. Start by locating the browser-extension popup and scrolling or input-handling entry point; compare mouse scrolling with the arrow-key behavior. Done means mouse scrolling moves the popup content without scrolling the main page.

Written by the indexing model from the issue text.