Function compute 

pub fn compute(widget: &Widget, avail_w: f32, avail_h: f32) -> LayoutNode

Compute layout starting at (0,0) with given available size. Returns the root LayoutNode with absolute coordinates.

Sizing rules (CSS-flexbox-like, single pass per container):

• A Units::Percent size resolves exactly once, against the parent’s content box (its size minus padding and scrollbar strip) on the same axis — in both a Row and a Column, at any depth. While a parent is still being measured for content on an axis (an auto size along an unbounded axis), a percent along that axis acts as Auto until the parent’s final size is known.
• flex (and Dock::Fill) grows a child into the parent’s free main-axis space; the grown size sticks and the child’s own children lay out against it. A growing child with an auto main size starts from its min size instead of its content (like WPF star sizing), so it fills exactly the free space — a scroll_view scrolls the rest, and nothing nested inside can push the child’s siblings out of the parent.