Created attachment 8897289 [details] Screen Shot 2017-08-15 at 3.03.21 pm.png User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20170814100258 Steps to reproduce: Created a WebExtension with a sidebar, and left the sidebar page's background color unset Actual results: The page's background is displayed as `transparent` in the inspector, but is drawn as a white background. Expected results: Much like how popups currently work on macOS, the sidebar page's background should be transparent, allowing the underlying UI (in this case the translucent sidebar background) to show through. This might be a macOS specific issue, but I don't have access to other platforms to verify this.
Component: Untriaged → WebExtensions: Frontend
Product: Firefox → Toolkit
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