User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-5739199-20150519190809/@comment-452-20150523135905

The purpose of Sandubadear's screenshots was to point out the loss of usable space on screens between 784 to 1023px.

Before this update, there was no margin, so the edit page used the full width - which was very convenient for people on netbooks.

While there may be ad-related motives for removing the margin on normal pages between 784px and 1023px, I think that the margin on the edit page should be changed back to how it is in the "normal" screenshot above.

(Of course, I have my CSS set to expand the editor like this at all resolutions)