From 22a117a091f2d5936533d361098e7483f2997568 Mon Sep 17 00:00:00 2001
From: Tobias Schmitz <tobiasschmitz2001@gmail.com>
Date: Mon, 12 May 2025 11:16:38 +0200
Subject: [PATCH] Prohibit some line break opportunities between LTR-ISOLATE
 and OBJECT-REPLACEMENT-CHARACTER (#6251)

Co-authored-by: Max <max@mkor.je>
Co-authored-by: Laurenz <laurmaedje@gmail.com>
---
 crates/typst-layout/src/inline/linebreak.rs   |  25 ++++++++++++++++--
 .../ref/issue-5489-matrix-stray-linebreak.png | Bin 0 -> 644 bytes
 tests/suite/layout/inline/linebreak.typ       |   8 ++++++
 3 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 tests/ref/issue-5489-matrix-stray-linebreak.png

diff --git a/crates/typst-layout/src/inline/linebreak.rs b/crates/typst-layout/src/inline/linebreak.rs
index 31512604f..ada048c7d 100644
--- a/crates/typst-layout/src/inline/linebreak.rs
+++ b/crates/typst-layout/src/inline/linebreak.rs
@@ -690,13 +690,34 @@ fn breakpoints(p: &Preparation, mut f: impl FnMut(usize, Breakpoint)) {
         let breakpoint = if point == text.len() {
             Breakpoint::Mandatory
         } else {
+            const OBJ_REPLACE: char = '\u{FFFC}';
             match lb.get(c) {
-                // Fix for: https://github.com/unicode-org/icu4x/issues/4146
-                LineBreak::Glue | LineBreak::WordJoiner | LineBreak::ZWJ => continue,
                 LineBreak::MandatoryBreak
                 | LineBreak::CarriageReturn
                 | LineBreak::LineFeed
                 | LineBreak::NextLine => Breakpoint::Mandatory,
+
+                // https://github.com/typst/typst/issues/5489
+                //
+                // OBJECT-REPLACEMENT-CHARACTERs provide Contingent Break
+                // opportunities before and after by default. This behaviour
+                // is however tailorable, see:
+                // https://www.unicode.org/reports/tr14/#CB
+                // https://www.unicode.org/reports/tr14/#TailorableBreakingRules
+                // https://www.unicode.org/reports/tr14/#LB20
+                //
+                // Don't provide a line breaking opportunity between a LTR-
+                // ISOLATE (or any other Combining Mark) and an OBJECT-
+                // REPLACEMENT-CHARACTER representing an inline item, if the
+                // LTR-ISOLATE could end up as the only character on the
+                // previous line.
+                LineBreak::CombiningMark
+                    if text[point..].starts_with(OBJ_REPLACE)
+                        && last + c.len_utf8() == point =>
+                {
+                    continue;
+                }
+
                 _ => Breakpoint::Normal,
             }
         };
diff --git a/tests/ref/issue-5489-matrix-stray-linebreak.png b/tests/ref/issue-5489-matrix-stray-linebreak.png
new file mode 100644
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diff --git a/tests/suite/layout/inline/linebreak.typ b/tests/suite/layout/inline/linebreak.typ
index e4b04b245..86a900252 100644
--- a/tests/suite/layout/inline/linebreak.typ
+++ b/tests/suite/layout/inline/linebreak.typ
@@ -139,3 +139,11 @@ Some texts feature many longer
 words. Those are often exceedingly
 challenging to break in a visually
 pleasing way.
+
+--- issue-5489-matrix-stray-linebreak ---
+#table(
+  columns: (70pt,) * 1,
+  align: horizon + center,
+  stroke: 0.6pt,
+  [$mat(2241/2210,-71/1105;-71/1105,147/1105)$],
+)