Fluent Community Content Spa
As you can see in this screenshot, we need the width of the content to push out to full length when there's no sidebar menu. This is really apparent when on the Feed view.
The reason we made the content width fixed (golden width that all social media sites use) is it's the perfect width for human eyes. Imagine, you have an image or video with full width or texts that expands the whole width, it will not look good.
Of course, you can use CSS to over-write it. But this is the default.
Ah I see! Thanks for the insights.