Constant DEFAULT_LAYER 

pub const DEFAULT_LAYER: u8 = _; // 3u8

PerfOverlayApp’s default overlay layer: Layer::Debug.

The workspace’s named top-most UI tier, so a perf HUD stays visible over whatever else is on screen (including an open Layer::Overlay popup) rather than risking a lower, app-chosen layer hiding it. Override with PerfOverlayApp::layer if an app’s own content already reaches this layer.