Fix improper line wrapping in the presence of medial newlines (#2056)

Fixes #2019.

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

@@ -1,3 +1,5 @@
+use std::iter::Peekable;
+
 use icu_properties::{maps::CodePointMapData, LineBreak};
 use icu_provider::AsDeserializingBufferProvider;
 use icu_provider_adapters::fork::ForkByKeyProvider;
@@ -933,18 +935,6 @@ fn linebreak_optimized<'a>(vt: &Vt, p: &'a Preparation<'a>, width: Abs) -> Vec<L
             // Layout the line.
             let start = pred.line.end;
 
-            // Fix for https://github.com/unicode-org/icu4x/issues/3811
-            if i > 0 {
-                if let Some(s_pred) = table.get(i + 1) {
-                    let next_start = s_pred.line.end;
-                    if !p.bidi.text[start..next_start]
-                        .contains(|c: char| !c.is_whitespace())
-                    {
-                        continue;
-                    }
-                }
-            }
-
             let attempt = line(vt, p, start..end, mandatory, hyphen);
 
             // Determine how much the line's spaces would need to be stretched
@@ -1103,7 +1093,7 @@ fn breakpoints<'a>(p: &'a Preparation<'a>) -> Breakpoints<'a> {
     linebreaks.next();
     Breakpoints {
         p,
-        linebreaks,
+        linebreaks: linebreaks.peekable(),
         syllables: None,
         offset: 0,
         suffix: 0,
@@ -1117,7 +1107,7 @@ struct Breakpoints<'a> {
     /// The paragraph's items.
     p: &'a Preparation<'a>,
     /// The inner iterator over the unicode line break opportunities.
-    linebreaks: LineBreakIteratorUtf8<'a, 'a>,
+    linebreaks: Peekable<LineBreakIteratorUtf8<'a, 'a>>,
     /// Iterator over syllables of the current word.
     syllables: Option<hypher::Syllables<'a>>,
     /// The current text offset.
@@ -1179,6 +1169,20 @@ impl Iterator for Breakpoints<'_> {
             }
         }
 
+        // Fix for https://github.com/unicode-org/icu4x/issues/3811
+        if !self.mandatory {
+            while let Some(&next) = self.linebreaks.peek() {
+                if !self.p.bidi.text[self.end..next]
+                    .contains(|c: char| !c.is_whitespace())
+                {
+                    self.end = next;
+                    self.linebreaks.next();
+                } else {
+                    break;
+                }
+            }
+        }
+
         self.offset = self.end;
         Some((self.end, self.mandatory, false))
     }

tests/typ/layout/par-justify.typ

@@ -50,3 +50,16 @@ This text can be fitted in one line.
 lorem ipsum 1234, lorem ipsum dolor sit amet
 
 #"  leading whitespace should still be displayed"
+
+---
+// Test that justification doesn't break code blocks
+
+#set par(justify: true)
+
+```cpp
+int main() {
+  printf("Hello world\n");
+  return 0;
+}
+```
+