Tor Browser 7.0 shows window resize warning on every new window for a few seconds

When I open a new Tor Browser window using Tor Browser 7.0 on macOS 10.12, I see a warning about resizing the window appear for a few seconds. Then it disappears.

This only happens when I open a new window with the default content (about:tor).

When I open a new empty tab, new tab with content, or new window with content, I don't see the warning.