Ah, good catch - I've fixed this in the attached. I also discovered that there's a parent-child hierarchy of WindowDefs (using relname->name), so instead of cloning the WindowDef (in parse_agg.c) if the frameOptions are different (e.g. by adding the ignore-nulls flag) I create a child of the WindowDef and override the frameOptions. This has the useful side-effect of making pg_get_viewdef work as expected (the previous iteration of the patch produced a copy of the window definintion, not the window name, as it was using a nameless clone), although the output has parentheses around the view name:
> lag(i.i, 2) IGNORE NULLS OVER (w) AS lagged_by_2