M4: editing the Dash of an odd-length dash pattern keeps its gaps (#1681)

An odd-length pattern is drawn repeated ([4, 2, 1] as [4, 2, 1, 4, 2, 1]),
so editing its Dash as three values changed the second gap. The pattern is
written out repeated first and only the edited entry changes.

crates/ui-egui/src/dash_pattern_tests.rs

@@ -53,3 +53,14 @@ fn a_single_value_pattern_is_the_dash_and_the_gap() {
     type_into(&mut u, "stroke_gap", "2");
     assert_eq!(dashes(&u, r), [5.0, 2.0]);
 }
+
+/// An odd-length pattern draws twice over (`[4, 2, 1]` is `[4, 2, 1, 4, 2, 1]`): editing its Dash
+/// writes the repeat out, so only the first dash changes and every gap stays as drawn.
+#[test]
+fn an_odd_length_pattern_keeps_its_repeat() {
+    let (mut u, r) = scene(json!([4, 2, 1]));
+    type_into(&mut u, "stroke_dash", "6");
+    assert_eq!(dashes(&u, r), [6.0, 2.0, 1.0, 4.0, 2.0, 1.0]);
+    type_into(&mut u, "stroke_gap", "3");
+    assert_eq!(dashes(&u, r), [6.0, 3.0, 1.0, 4.0, 2.0, 1.0]);
+}

crates/ui-egui/src/panels/right/stroke.rs

@@ -189,12 +189,13 @@ fn brush_row(app: &mut FigApp, ui: &mut Ui, ids: &[NodeId], brush: Option<Option
 
 /// Dash pattern `dashes` with its first dash (`i` 0) or first gap (`i` 1) set to `v`, keeping the
 /// rest of the pattern: the panel shows only the first pair, and editing it must not collapse an
-/// imported `[4, 2, 1, 2]` to two values (#1681). A single value is the dash and the gap alike.
+/// imported `[4, 2, 1, 2]` to two values (#1681). An odd-length pattern is drawn repeated
+/// (`figcraft_pathops::width::dash_pattern`: `[4, 2, 1]` is `[4, 2, 1, 4, 2, 1]`), so it is
+/// written out that way first and only the edited entry changes; `[5]` becomes `[5, 5]`.
 fn set_dash(dashes: &[f64], i: usize, v: f64) -> Vec<f64> {
-    let mut d = dashes.to_vec();
-    if d.len() < 2 {
-        let first = d.first().copied().unwrap_or(4.0);
-        d = vec![first, first];
+    let mut d = if dashes.is_empty() { vec![4.0] } else { dashes.to_vec() };
+    if d.len() % 2 == 1 {
+        d = d.repeat(2);
     }
     if let Some(x) = d.get_mut(i) {
         *x = v.max(0.0);