Fix layout panic in rounded rectangle

Fixes #1216 both by hardening the rounded rectangle against negative size and by ensuring that the negative size doesn't occur in the first place for this case.

library/src/layout/flow.rs

@@ -243,6 +243,11 @@ impl<'a> FlowLayouter<'a> {
         block: &Content,
         styles: StyleChain,
     ) -> SourceResult<()> {
+        // Skip directly if regino is already full.
+        if self.regions.is_full() {
+            self.finish_region()?;
+        }
+
         // Placed elements that are out of flow produce placed items which
         // aren't aligned later.
         if let Some(placed) = block.to::<PlaceElem>() {

src/geom/rounded.rs

@@ -52,7 +52,7 @@ fn stroke_segments(
     let mut connection = Connection::default();
     let mut path = Path::new();
     let mut always_continuous = true;
-    let max_radius = size.x.min(size.y) / 2.0;
+    let max_radius = size.x.min(size.y).max(Abs::zero()) / 2.0;
 
     for side in [Side::Top, Side::Right, Side::Bottom, Side::Left] {
         let continuous = stroke.get_ref(side) == stroke.get_ref(side.next_cw());

tests/typ/bugs/clamp-panic.typ

@@ -0,0 +1,3 @@
+#set page(height: 20pt, margin: 0pt)
+#v(22pt)
+#block(fill: red, width: 100%, height: 10pt, radius: 4pt)