Beta 12: composition mode discrepancy [Bug?]

I have been using composition mode a lot recently and I have noticed a disturbing bug (or I have failed to find a setting!).

If I open a document and then open composition mode I can see the text and it has reasonable margins on both left and right. It works just as I would expect.

If I click on a folder and then enter composition mode, I see the content of that folder as scrivenings in composition mode. However the text now has no margins at all. This is particularly disturbing on the left hand side where every line starts flush to the left of the window.

Is the behaviour supposed to be different in these two cases, is there a setting I should adjust, or is this a bug? I would expect the left and right margins to show in both cases (since without them composition mode makes reading harder rather than easier).