Fix duplicate state manipulation

The flow layouts footnotes twice in some cases, which messed up the Vt's locator state. Typically, we use `thing.measure` instead of `thing.layout` to prevent side effects, but in this case it was simpler to just undo the modification. A future layout engine rewrite should try to make this kind of error harder to make.

Fixes #1597

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

@@ -617,6 +617,7 @@ impl FlowLayouter<'_> {
             }
 
             self.regions.size.y -= self.footnote_config.gap;
+            let checkpoint = vt.locator.clone();
             let frames = FootnoteEntry::new(notes[k].clone())
                 .pack()
                 .layout(vt, self.styles, self.regions.with_root(false))?
@@ -637,6 +638,9 @@ impl FlowLayouter<'_> {
                     self.regions.size.y -= item.height();
                 }
 
+                // Undo Vt modifications.
+                *vt.locator = checkpoint;
+
                 return Ok(false);
             }
 

crates/typst/src/model/introspect.rs

@@ -111,7 +111,7 @@ cast! {
 /// [^1]: Well, we could with [`TrackedMut`](comemo::TrackedMut), but the
 /// overhead is quite high, especially since we need to save & undo the counting
 /// when only measuring.
-#[derive(Default)]
+#[derive(Default, Clone)]
 pub struct Locator<'a> {
     /// Maps from a hash to the maximum number we've seen for this hash. This
     /// number becomes the `disambiguator`.