Function gutters_frame_table 

fn gutters_frame_table(g: &GutterRects) -> bool

Whether the gutters genuinely frame the table: the row gutter’s right edge is the table’s left edge, the column gutter’s bottom edge is the table’s top edge, each gutter spans the table along its other axis, and the corner well sits below the table at the row gutter’s left edge.

This is the geometric property the old horizontal “axis shelf” failed: three drop zones side by side above the grid touch no grid edge at all. It is debug_assert!ed every frame (when the window has room — see gutters_have_room) and asserted on recorded rects in the headless layout test.