How do I resize the window? The normal edges don't seem to grab.

Drag any edge or corner like a normal app. The app draws its own resize handles over the border, and they're actually a bit WIDER than a normal window edge. Aim for the very outer rim (the thin dark border around the white card) and drag — all four edges and corners work. If dragging genuinely does nothing, the window is maximized (resizing is disabled while maximized) — click the green button to come out of maximized first.