Fix floating placement within containers

Fixes #1879

crates/typst-library/src/layout/flow.rs

@@ -474,7 +474,7 @@ impl<'a> FlowLayouter<'a> {
                 }
             }
         }
-        used.y += footnote_height;
+        used.y += footnote_height + float_top_height + float_bottom_height;
 
         // Determine the size of the flow in this region depending on whether
         // the region expands. Also account for fractional spacing and

tests/typ/layout/place-float-columns.typ

@@ -0,0 +1,19 @@
+// Test floats in columns.
+
+---
+#set page(height: 200pt, width: 300pt)
+#show: columns.with(2)
+
+= Introduction
+#figure(
+  placement: bottom,
+  caption: [A glacier],
+  image("/files/glacier.jpg", width: 50%),
+)
+#lorem(45)
+#figure(
+  placement: top,
+  caption: [A rectangle],
+  rect[Hello!],
+)
+#lorem(20)