Implement the `leading` attribute as defined by fonts.

`EditText` supports two different forms of leading:

1. Font-provided leading, specified relative to the EM square and scaled with font size
2. User-specificed leading, specified in pixels

Notably, the former appears to apply to the first line in the text and pushes it down. This showed up in the `edittext_font_size` test, and according to that test result the leading is rounded *up* to the nearest pixel, plus one.

That last bit seems possibly wrong and is subject to further change, but it matches the tests at multiple scales.

core/src/font.rs

@@ -3,11 +3,16 @@ use crate::prelude::*;
 use crate::transform::Transform;
 use gc_arena::{Collect, Gc, MutationContext};
 
-/// Certain Flash routines measure text up to the nearest whole pixel.
-fn round_to_pixel(t: Twips) -> Twips {
+/// Certain Flash routines measure text by rounding down to the nearest whole pixel.
+fn round_down_to_pixel(t: Twips) -> Twips {
     Twips::from_pixels(t.to_pixels().floor())
 }
 
+/// Certain Flash routines measure text by rounding up to the nearest whole pixel.
+pub fn round_up_to_pixel(t: Twips) -> Twips {
+    Twips::from_pixels(t.to_pixels().ceil())
+}
+
 type Error = Box<dyn std::error::Error>;
 
 #[derive(Debug, Clone, Collect, Copy)]
@@ -33,6 +38,18 @@ struct FontData {
     /// Maps from a pair of unicode code points to horizontal offset value.
     kerning_pairs: fnv::FnvHashMap<(u16, u16), Twips>,
 
+    /// The distance from the top of each glyph to the baseline of the font, in
+    /// EM-square coordinates.
+    ascent: u16,
+
+    /// The distance from the baseline of the font to the bottom of each glyph,
+    /// in EM-square coordinates.
+    descent: u16,
+
+    /// The distance between the bottom of any one glyph and the top of
+    /// another, in EM-square coordinates.
+    leading: i16,
+
     /// The identity of the font.
     descriptor: FontDescriptor,
 }
@@ -65,6 +82,11 @@ impl<'gc> Font<'gc> {
         };
 
         let descriptor = FontDescriptor::from_swf_tag(tag);
+        let (ascent, descent, leading) = if let Some(layout) = &tag.layout {
+            (layout.ascent, layout.descent, layout.leading)
+        } else {
+            (0, 0, 0)
+        };
 
         Ok(Font(Gc::allocate(
             gc_context,
@@ -76,6 +98,9 @@ impl<'gc> Font<'gc> {
                 /// (SWF19 p.164)
                 scale: if tag.version >= 3 { 20480.0 } else { 1024.0 },
                 kerning_pairs,
+                ascent,
+                descent,
+                leading,
                 descriptor,
             },
         )))
@@ -119,6 +144,13 @@ impl<'gc> Font<'gc> {
             .unwrap_or_default()
     }
 
+    /// Return the leading for this font at a given height.
+    pub fn get_leading_for_height(self, height: Twips) -> Twips {
+        let scale = height.get() as f32 / self.scale();
+
+        Twips::new((self.0.leading as f32 * scale) as i32)
+    }
+
     /// Returns whether this font contains kerning information.
     pub fn has_kerning_info(self) -> bool {
         !self.0.kerning_pairs.is_empty()
@@ -179,8 +211,8 @@ impl<'gc> Font<'gc> {
             |transform, _glyph, advance| {
                 let tx = transform.matrix.tx;
                 let ty = transform.matrix.ty;
-                size.0 = std::cmp::max(size.0, round_to_pixel(tx + advance));
-                size.1 = std::cmp::max(size.1, round_to_pixel(ty));
+                size.0 = std::cmp::max(size.0, round_down_to_pixel(tx + advance));
+                size.1 = std::cmp::max(size.1, round_down_to_pixel(ty));
             },
         );
 

core/src/html/layout.rs

@@ -1,7 +1,7 @@
 //! Layout box structure
 
 use crate::context::UpdateContext;
-use crate::font::Font;
+use crate::font::{round_up_to_pixel, Font};
 use crate::html::dimensions::{BoxBounds, Position, Size};
 use crate::html::text_format::{FormatSpans, TextFormat, TextSpan};
 use crate::tag_utils::SwfMovie;
@@ -117,6 +117,14 @@ impl<'a, 'gc> LayoutContext<'a, 'gc> {
             Twips::from_pixels(0.0),
         );
 
+        // Flash appears to round up the font's leading to the nearest pixel
+        // and adds one. I'm not sure why.
+        let font_leading_adjustment = round_up_to_pixel(
+            self.font
+                .map(|f| f.get_leading_for_height(self.max_font_size))
+                .unwrap_or_else(|| Twips::new(0)),
+        ) + Twips::from_pixels(1.0);
+
         line = self.current_line;
         while let Some(linebox) = line {
             let mut write = linebox.write(mc);
@@ -126,8 +134,10 @@ impl<'a, 'gc> LayoutContext<'a, 'gc> {
             // which is information we don't have yet.
             let font_size_adjustment = self.max_font_size - write.bounds.height();
 
-            write.bounds +=
-                Position::from((left_adjustment + align_adjustment, font_size_adjustment));
+            write.bounds += Position::from((
+                left_adjustment + align_adjustment,
+                font_size_adjustment + font_leading_adjustment,
+            ));
             line = write.next_sibling();
         }
 

core/tests/regression_tests.rs

@@ -197,6 +197,7 @@ swf_tests! {
     (as1_constructor_v6, "avm1/as1_constructor_v6", 1),
     (as1_constructor_v7, "avm1/as1_constructor_v7", 1),
     (issue_710, "avm1/issue_710", 1),
+    (edittext_font_size, "avm1/edittext_font_size", 1),
 }
 
 // TODO: These tests have some inaccuracies currently, so we use approx_eq to test that numeric values are close enough.

core/tests/swfs/avm1/edittext_font_size/output.txt

@@ -0,0 +1,45 @@
+//Creating left aligned text @ 10px
+W: 7
+H: 15
+//Creating left aligned text @ 12px
+W: 9
+H: 18
+//Creating left aligned text @ 20px
+W: 15
+H: 29
+//Creating left aligned text @ 30px
+W: 23
+H: 42
+//Creating left aligned text @ 100px
+W: 78
+H: 138
+//Creating left aligned text 'Qe' @ 10px
+W: 13
+H: 15
+//Creating left aligned text 'Qe' @ 12px
+W: 16
+H: 18
+//Creating left aligned text 'Qe' @ 20px
+W: 26
+H: 29
+//Creating left aligned text 'Qe' @ 30px
+W: 40
+H: 42
+//Creating left aligned text 'Qe' @ 100px
+W: 134
+H: 138
+//Creating left aligned text 'Q e' @ 10px
+W: 15
+H: 15
+//Creating left aligned text 'Q e' @ 12px
+W: 19
+H: 18
+//Creating left aligned text 'Q e' @ 20px
+W: 32
+H: 29
+//Creating left aligned text 'Q e' @ 30px
+W: 48
+H: 42
+//Creating left aligned text 'Q e' @ 100px
+W: 160
+H: 138