core: Use correct side for top margin calculation

The fallback to the smaller page should only occure when the margin is
`auto`, but unconditionally used the smaller page beforehand.

Closes #31.

Author: tingerrr

src/core.typ

@@ -28,7 +28,7 @@
 
   // `+ 0%` forces this to be a relative length
   margin = util.auto-or(margin, () => (2.5 / 21) * min) + 0%
-  margin.length.to-absolute() + (min * margin.ratio)
+  margin.length.to-absolute() + (height * margin.ratio)
 }
 
 /// #property(requires-context: true)

tests/regressions/precentage-margin/.gitignore

@@ -0,0 +1,4 @@
+# generated by tytanic, do not edit
+
+diff/**
+out/**

tests/regressions/precentage-margin/ref/1.png

@@ -0,0 +1,22 @@
+/// Synopsis:
+/// - Tracked in #31.
+/// - The smallest page should only be used for the `2.5 / 21` fallback.
+
+#import "/src/lib.typ": hydra
+
+#set page(
+  paper: "a4",
+  margin: (top: 10%),
+  header: context hydra(1, skip-starting: false),
+)
+
+#show heading.where(level: 1): it => pagebreak(weak: true) + it
+
+= Chapter 1
+#lorem(100)
+
+= Chapter 2
+#lorem(200)
+
+= Chapter 3
+#lorem(200)