Text: line spacing reduction comes off the percentage, not the spacing

PowerPoint's lnSpcReduction takes percentage points off a paragraph's line
spacing (90 % less 20 % is 70 %) and leaves the space before and after
paragraphs alone. We multiplied (90 % x 0.8 = 72 %) and shrank the
paragraph spacing too, so shrunk text came out with different line and
paragraph pitches than PowerPoint's: measured on PowerPoint's PDF of a real
deck, a body at 90 % spacing, 10 pt before paragraphs and a 20 % reduction
runs at 20.2 pt lines with 9.9 pt between paragraphs.

crates/text/src/lib.rs

@@ -320,10 +320,12 @@ fn face_chain(ctx: &Ctx, st: &Style, slot: FontSlot, script: Option<ScriptTag>)
     chain
 }
 
-fn spacing_pts(s: Option<Spacing>, size: f64, reduce: f64) -> f64 {
+/// Space before or after a paragraph, points. Shrink-on-overflow's line spacing reduction
+/// doesn't apply to it (PowerPoint keeps it).
+fn spacing_pts(s: Option<Spacing>, size: f64) -> f64 {
     match s {
-        Some(Spacing::Pct(p)) => p * size * 1.2 * (1.0 - reduce),
-        Some(Spacing::Pts(v)) => v * (1.0 - reduce),
+        Some(Spacing::Pct(p)) => p * size * 1.2,
+        Some(Spacing::Pts(v)) => v,
         None => 0.0,
     }
 }
@@ -918,7 +920,7 @@ fn layout_scaled(ctx: &Ctx, shape: &Shape, body: &TextBody, bp: &BodyProps, opts
         let first_w = (col_w - first_x - mr).max(1.0);
         let rest_w = (col_w - rest_x - mr).max(1.0);
         let raw = break_lines(p, first_w, rest_w, wrap, &tabs, props.default_tab.unwrap_or(72.0), first_x, rest_x);
-        let before = if pi == 0 { 0.0 } else { spacing_pts(props.space_before, p.lead_size, reduce) };
+        let before = if pi == 0 { 0.0 } else { spacing_pts(props.space_before, p.lead_size) };
         y += before.max(0.0) + prev_after;
         let first_line_idx = all_lines.len();
         for (li, rl) in raw.iter().enumerate() {
@@ -941,7 +943,9 @@ fn layout_scaled(ctx: &Ctx, shape: &Shape, body: &TextBody, bp: &BodyProps, opts
             }
             let natural = (asc + desc).max(size * 1.0);
             let lh = match props.line_spacing {
-                Some(Spacing::Pct(f)) => natural * f.max(0.0) * (1.0 - reduce),
+                // The reduction comes off the percentage (90 % less 20 % is 70 %), as PowerPoint
+                // applies `lnSpcReduction`.
+                Some(Spacing::Pct(f)) => natural * (f - reduce).max(f * 0.1).max(0.0),
                 Some(Spacing::Pts(v)) => v * (1.0 - reduce),
                 None => natural * (1.0 - reduce),
             };
@@ -1082,7 +1086,7 @@ fn layout_scaled(ctx: &Ctx, shape: &Shape, body: &TextBody, bp: &BodyProps, opts
             ));
         }
         out.para_lines.push((first_line_idx, raw.len()));
-        prev_after = spacing_pts(props.space_after, p.lead_size, reduce).max(0.0);
+        prev_after = spacing_pts(props.space_after, p.lead_size).max(0.0);
     }
     let total = y;
     out.content_height = total;

crates/text/src/tests.rs

@@ -54,6 +54,37 @@ fn shrink_on_overflow() {
     assert!(!l.overflow || l.font_scale <= 0.11);
 }
 
+#[test]
+fn line_spacing_reduction_comes_off_the_percentage() {
+    // 90 % line spacing and 10 pt before paragraphs, shrunk with a 20 % line spacing reduction:
+    // PowerPoint draws 70 % lines and keeps the 10 pt.
+    let (p, mut sh) = setup("One\nTwo\nThree", 600.0, 400.0);
+    for (i, para) in sh.text.as_mut().unwrap().paragraphs.iter_mut().enumerate() {
+        para.props.line_spacing = Some(Spacing::Pct(0.9));
+        para.props.space_before = Some(Spacing::Pts(10.0));
+        para.props.bullet = Some(Bullet::None);
+        if i == 0 {
+            para.runs[0].text.push_str(" wraps onto a second line because this paragraph is long enough to wrap in a narrow box");
+        }
+    }
+    let ctx = Ctx::for_slide(&p, &p.slides[0]).unwrap();
+    let body = sh.text.clone().unwrap();
+    let bp = resolve::body(&ctx, &sh);
+    let opts = Opts { rect: Rect::new(0.0, 0.0, 200.0, 400.0), fields: &NoFields, prompt_color: None, no_shrink: true };
+    let full = layout_scaled(&ctx, &sh, &body, &bp, &opts, 1.0, 0.0);
+    let reduced = layout_scaled(&ctx, &sh, &body, &bp, &opts, 1.0, 0.2);
+    let pitch = |l: &TextLayout| l.lines[1].top - l.lines[0].top;
+    // Lines within the first paragraph: 0.7 / 0.9 of the full pitch.
+    assert!(full.lines[1].para == 0 && reduced.lines[1].para == 0);
+    assert!((pitch(&reduced) / pitch(&full) - 0.7 / 0.9).abs() < 0.005, "{} {}", pitch(&reduced), pitch(&full));
+    // The space before the next paragraph is the same 10 pt.
+    let gap = |l: &TextLayout| {
+        let k = l.lines.iter().position(|x| x.para == 1).unwrap();
+        l.lines[k].top - l.lines[k - 1].bottom
+    };
+    assert!((gap(&full) - 10.0).abs() < 1e-6 && (gap(&reduced) - 10.0).abs() < 1e-6, "{} {}", gap(&full), gap(&reduced));
+}
+
 #[test]
 fn caret_and_hit_roundtrip() {
     let (p, sh) = setup("Hello world", 600.0, 100.0);