Fix tags at the start of paragraphs (#4978)

Co-authored-by: Martin Haug <mhaug@live.de>

crates/typst/src/layout/inline/prepare.rs

@@ -58,7 +58,13 @@ impl<'a> Preparation<'a> {
 
     /// Iterate over the items that intersect the given `sliced` range.
     pub fn slice(&self, sliced: Range) -> impl Iterator<Item = &(Range, Item<'a>)> {
-        let start = self.indices.get(sliced.start).copied().unwrap_or(0);
+        // Usually, we don't want empty-range items at the start of the line
+        // (because they will be part of the previous line), but for the first
+        // line, we need to keep them.
+        let start = match sliced.start {
+            0 => 0,
+            n => self.indices.get(n).copied().unwrap_or(0),
+        };
         self.items[start..].iter().take_while(move |(range, _)| {
             range.start < sliced.end || range.end <= sliced.end
         })

tests/suite/model/par.typ

@@ -101,3 +101,8 @@ Lorem ipsum dolor #metadata(none) nonumy eirmod tempor.
 --- issue-4938-par-bad-ratio ---
 #set par(justify: true)
 #box($k in NN_0$)
+
+--- issue-4770-par-tag-at-start ---
+#h(0pt) #box[] <a>
+
+#context test(query(<a>).len(), 1)