Constant WINDOW_EARLY_FRACTION 

const WINDOW_EARLY_FRACTION: f64 = 0.25;

The share of a frame period before the next window opens in which a change, paced in windows, takes that window’s frame at once rather than waiting for it to open. One frame a window still holds the rate at the cadence; a source drawing at the cadence whose frames end just short of the boundary otherwise waits for it every frame (a 60 fps full-screen animation on Xvfb waited 0.8 ms a frame on average, in some phases of its clock against the windows’).