Fix #5111: keep coloured harakat attached when the first letter is spaced

paint_only ignores kerning_before, as drawn_alike does: a space before
a line's first letter, set on the letter alone, no longer splits it from
its harakat coloured apart.

crates/text/src/harakat.rs

@@ -38,7 +38,8 @@ pub(crate) fn is_harakat(c: char) -> bool {
 
 /// Do character styles `a` and `b` differ, if at all, only in paint: fill, stroke and how the
 /// stroke is drawn, overprint, and the character style name? Those are not read by the shaper
-/// and change no glyph, advance or position. Stricter than [`crate::shape`]'s `shapes_alike`
+/// and change no glyph, advance or position. Nor is the space before a line's first character
+/// inside a cluster, which takes it from its first character (#5111), so it is ignored too. Stricter than [`crate::shape`]'s `shapes_alike`
 /// (which also ignores baseline shift, tracking, underline, …).
 pub(crate) fn paint_only(a: &CharStyle, b: &CharStyle) -> bool {
     // Exhaustive: a new attribute must explicitly keep or relax this boundary rule.
@@ -51,7 +52,7 @@ pub(crate) fn paint_only(a: &CharStyle, b: &CharStyle) -> bool {
         leading,
         tracking,
         kerning,
-        kerning_before,
+        kerning_before: _,
         optical_kerning,
         baseline_shift,
         h_scale,
@@ -101,7 +102,6 @@ pub(crate) fn paint_only(a: &CharStyle, b: &CharStyle) -> bool {
         && leading == &b.leading
         && tracking == &b.tracking
         && kerning == &b.kerning
-        && kerning_before == &b.kerning_before
         && optical_kerning == &b.optical_kerning
         && baseline_shift == &b.baseline_shift
         && h_scale == &b.h_scale

crates/text/src/tests/harakat.rs

@@ -171,6 +171,7 @@ fn paint_only_keeps_every_nonpaint_attribute_and_partial_equality() {
         b.stroke_dash = a.stroke_dash.clone();
         b.stroke_align = a.stroke_align;
         b.style_name = a.style_name.clone();
+        b.kerning_before = a.kerning_before;
         *a == b
     }
 
@@ -203,6 +204,7 @@ fn paint_only_keeps_every_nonpaint_attribute_and_partial_equality() {
     field!(leading, Some(60.0), false);
     field!(tracking, 20.0, false);
     field!(kerning, Some(0.0), false);
+    field!(kerning_before, 250.0, true);
     field!(optical_kerning, true, false);
     field!(baseline_shift, 1.0, false);
     field!(h_scale, 90.0, false);
@@ -312,6 +314,26 @@ fn harakat_ranges_normalise_their_range() {
 /// the same way, and only a character in that glyph's shaping cluster: repeated glyph ids alone
 /// don't do. A later in-cluster owner is kept only for a GDEF mark glyph on a harakat character;
 /// everything else stays at the cluster start (conservative).
+/// A space before the line's first letter (#5111), set on the letter alone as the Type tool's caret
+/// sets it, keeps the letter's harakat coloured apart attached and in their own colour. Right to
+/// left the space is past the letter's right side, so every glyph stays where it was.
+#[test]
+fn a_space_before_the_first_letter_keeps_its_coloured_harakat() {
+    let Some(face) = arabic_face("a_space_before_the_first_letter_keeps_its_coloured_harakat") else {
+        return;
+    };
+    const WORD: &str = "بَبَب"; // ب 0, fatha 2, ب 4, fatha 6, ب 8
+    let db = FontDb::global();
+    let coloured = text_with_harakat(WORD, &face.family, Some(red()));
+    let mut spaced = coloured.clone();
+    crate::edit::style_range(&mut spaced.runs, 0, 2, |st| st.kerning_before = 250.0);
+    let (a, b) = (layout(db, &coloured), layout(db, &spaced));
+    let placed = |l: &TextLayout| l.glyphs.iter().map(|g| (g.font_id, g.gid, g.byte, g.xf.as_coeffs().map(f64::to_bits))).collect::<Vec<_>>();
+    assert_eq!(placed(&a), placed(&b), "the same glyphs where they were");
+    assert_eq!(painted(&spaced, &b, &red()), painted(&coloured, &a, &red()), "the harakat keep their colour");
+    assert!(painted(&spaced, &b, &red()) > 0, "harakat painted apart");
+}
+
 #[test]
 fn own_chars_need_the_same_setting_and_each_character_in_its_cluster() {
     // بَبَب right to left: final beh (cluster 8), fatha and medial beh (cluster 4), fatha and