Created attachment 169927 [details] File with gutter an a proposed guide line, here dashed The feature "page gutter" was implemented with bug 91920. It allows the user to distinguish in the margins between an area, which will be consumed by binding and an area, which gives a visible margin. But outside the page style dialog this is not visible. So the user can easily put things into the page gutter area, which then will be hidden by binding. Enhancement request: If the option "Text Boundaries" is enabled (menu View), then a guide line should be drawn between gutter area and area of true margin.
I added keyword "needsUXEval", because it's an enhancement request.
Yes, it's kind of a boundary and would be shown if View > Text boundary is checked. Could be either a dotted line or the whole gutter becomes hatched out to align with the preview at the page style dialog, see bug 140581.