diff --git a/core/src/orientation/pos.rs b/core/src/orientation/pos.rs
index 28fd20a..495575b 100644
--- a/core/src/orientation/pos.rs
+++ b/core/src/orientation/pos.rs
@@ -282,7 +282,26 @@ impl UiSpan {
         self.end += offset;
     }
 
+    /// The whole of the box it sits in: a span that composes to nothing and
+    /// a parent that changes nothing.
+    pub const fn is_full(&self) -> bool {
+        self.start.rel.raw() == Rel::ZERO.raw()
+            && self.start.px.raw() == Px::ZERO.raw()
+            && self.end.rel.raw() == Rel::ONE.raw()
+            && self.end.px.raw() == Px::ZERO.raw()
+    }
+
     pub const fn within(&self, parent: &Self) -> Self {
+        // A part that is the whole box is the box, and a box composed through
+        // the whole of its parent is itself. Both are exact -- multiplying by
+        // one rounds to what it started as -- and both are common enough to
+        // be worth four comparisons rather than four multiplies to find out.
+        if self.is_full() {
+            return *parent;
+        }
+        if parent.is_full() {
+            return *self;
+        }
         Self {
             start: self.start.within(parent),
             end: self.end.within(parent),