Revise block node contract

Frames produced by block nodes are now always treated as exactly one per
given region and a frame must not be larger than its respective region.
Any overflow must be handled internally. This means that stack and grid
don't need to search for fitting regions anymore, since the child has
already does that for them. This commit further moves stack spacing into
a new `SpacingNode`.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/layout/shape.rs b/src/layout/shape.rs
index aa90707c..3469f660 100644
--- a/src/layout/shape.rs
+++ b/src/layout/shape.rs
@@ -46,7 +46,8 @@ impl Layout for ShapeNode {
             let mut cts = Constraints::new(regions.expand);
             cts.set_base_if_linear(regions.base, Spec::new(self.width, self.height));
 
-            // Set exact and base constraint if child is automatically sized.
+            // Set tight exact and base constraints if the child is
+            // automatically sized since we don't know what the child might do.
             if self.width.is_none() {
                 cts.exact.x = Some(regions.current.w);
                 cts.base.x = Some(regions.base.w);