In FireFox extensions, the Panel/Popup that opens on the Toolbar sizes itself outside the browser window, if needed, so that we see every populated elements in the panel. In Chrome however, the popup/panel is only drawn until the browser window's boundaries. So, if the user resizes the browser window small enough, you don't see the entire popup.
I checked the documentation and couldn't find anything. Is there anything that can be done to show the entire popup?
This seems to be OS-dependent (can be reproduced on Linux and Win7, but not Win10).
As an extension author, there is nothing you can do to control it, this is just how the browser renders its content. You could submit a bug report.