Bugzilla

We anticipated this issue and looked into it in bug 1893214. In summary, our documentation has long advised theme authors that they can place one or more background images behind the toolbars at the top of the browser window. We even suggest a size, typically 2000px wide and at least [200px high](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme#images). The new sidebar adds a sidebar launcher - which is *kind of* a toolbar - below the other toolbars and on the side of the content area. Ideally we'd enable the background image to sit behind the sidebar launcher as well. But most of even all of the existing 2000x200 size and aspect ratio theme images would not work well there. They were never designed to stretch or tile vertically. Another option would be to add the theme's background image to the sidebar so it would show up twice. That might work well for some themes and not so well for others. 

The design intention was always to have a smooth seamless transition between the horiztonal toolbars and the new launcher vertical toolbar. To support that I think we may have to extend the theme manifest itself and add a new property for images explicitly design to work this way.

I looked for and failed to find a bug on file for that work. Unless I just missed it, maybe this can be it?

We anticipated this issue and looked into it in bug 1893214. In summary, our documentation has long advised theme authors that they can place one or more background images behind the toolbars at the top of the browser window. We even suggest a size, typically 2000px wide and at least [200px high](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme#images). The new sidebar adds a sidebar launcher - which is *kind of* a toolbar - below the other toolbars and on the side of the content area. Ideally we'd enable the background image to sit behind the sidebar launcher as well. But most of the existing 2000x200 size and aspect ratio theme images would not work well there. They were never designed to stretch or tile vertically. Another option would be to add the theme's background image to the sidebar so it would show up twice. That might work well for some themes and not so well for others. 

The design intention was always to have a smooth seamless transition between the horizontal toolbars and the new launcher vertical toolbar. To support that I think we may have to extend the theme manifest itself and add a new property for images explicitly design to work this way.

I looked for and failed to find a bug on file for that work. Unless I just missed it, maybe this can be it?