Function gutters_have_room 

fn gutters_have_room(g: &GutterRects) -> bool

Whether the window left the gutters enough room to be laid out at all.

A panel clamps its extent to what is available, so in an absurdly small window (a few dozen points of central panel) the bottom well is squeezed up past the table and gutters_frame_table cannot hold — through no fault of the layout. The per-frame debug_assert! is gated on this so shrinking the window can never panic a debug build; the invariant itself stays strict, and the layout test asserts it at a real window size.