#z-index

ScreenPanel.ZIndex defaults to 100 (not 0), so any ScreenPanel that never sets a tier collides with the tier most projects author their modals at, and two roots with equal ZIndex have no defined paint order at all. Cross-root compositing is a stable OrderBy over an unordered HashSet, so tied roots keep an enumeration order that matches creation on a fresh boot but re-rolls on any enable/disable interleave or editor hotload. A full-screen surface can paint a perfectly healthy tree and still be invisible under a tied root.

ScreenPanel.ZIndex orders paint only: pointer input is NOT routed by cross-root stacking, so a lower-ZIndex root keeps swallowing clicks even when a higher-ZIndex modal draws over it.