When you open a topic in the XML Editor, a default (not "Default", as in the name of that particular layout) is applied automatically. This is distinct from the page layout applied to it in the target.
So for example, if I open a topic to edit, a Copyright.flpgl layout is applied.
Is this simply chosen because it is the first alphabetically? That is, if I want the Default.flpgl layout to be automatically applied when I open a topic to edit in the XML Editor, I should rename Copyright.flpgl to a letter after D - say, "Title Copyright.flpgl"?
EDIT: Ah, discovered I can assign a default page layout for the project as so:
1. In Project Explorer, right-click on the root project name and select "Project Properties..."
2. In the Defaults tab, select the required Master Page Layout (for example, Resources/PageLayouts/Default) and click OK.
P.