fix(format): bound the characters Sketch text runs write in all

crates/format/src/sketch/text.rs

@@ -14,6 +14,9 @@ use super::tree::{Cx, color};
 const MAX_CHARS: usize = 1 << 20;
 /// Most runs read from one text layer.
 const MAX_RUNS: usize = 10_000;
+/// Most characters that styled runs may write in all, overlaps counted again (a run's range is
+/// rewritten in full, even where later runs overwrite it).
+const MAX_RUN_WORK: usize = 16 * MAX_CHARS;
 
 /// Fill `node` (a TEXT node) from text layer `l`.
 pub(crate) fn node(cx: &mut Cx, l: &Value, node: &mut JsonMap<String, Value>) {
@@ -80,10 +83,16 @@ pub(crate) fn node(cx: &mut Cx, l: &Value, node: &mut JsonMap<String, Value>) {
     // Runs that differ from the first: a style id per UTF-16 unit.
     let mut ids = vec![0u64; units];
     let mut table = JsonMap::new();
+    let mut work = 0usize;
     for (start, end, s) in &styles {
         if *s == base {
             continue;
         }
+        work = work.saturating_add(end.saturating_sub(*start));
+        if work > MAX_RUN_WORK {
+            cx.warn("text whose styled runs overlap a great deal keeps the first runs' styles");
+            break;
+        }
         let id = table.len() as u64 + 1;
         table.insert(id.to_string(), s.clone());
         for slot in ids.get_mut(*start..*end).into_iter().flatten() {

crates/format/src/sketch/text_tests.rs

@@ -271,3 +271,13 @@ fn later_runs_dont_inherit_the_first_runs_settings() {
     assert_eq!(second.line_height, figcraft_doc::LineHeight::Auto, "{second:?}");
     assert!(second.variations.is_empty(), "{second:?}");
 }
+
+#[test]
+fn runs_that_overlap_a_great_deal_stop_being_applied() {
+    // 17 runs over the whole of a text of 2^20 characters: 17 × 2^20 writes without the limit.
+    let n = 1 << 20;
+    let mut runs = vec![(0, 1, "Helvetica", 10.0, BLACK)];
+    runs.extend((0..17).map(|i| (0, n, "Helvetica", 11.0 + f64::from(i), BLACK)));
+    let (_, warnings) = open(vec![text("Long", 0.0, 0.0, 200.0, 20.0, &"a".repeat(n), &runs)]);
+    assert!(warnings.iter().any(|w| w.contains("overlap a great deal")), "{warnings:?}");
+}