[rjwse@aol.com]

I have been using calibre's "viewer" to evaluate media queries which respond to total width of entire screen. I would first view narrow (as for phone) then widen the "viewer" (as for a wide tablet). I never noticed it before, but to my delight the editor's "file preview" also functions as though it were the entire screen! It is easier for me now to double check response narrow/wide just in the editor and I do not even have to open the "viewer."