text: add line-height property

Expose a font-size-relative line height on Text, TextInput, and StyledText.

Apply it consistently in Parley, testing, and software-renderer layout, including measurement, elision, and cursor positioning.

Document the property and cover the layout and hit-testing behavior.

ChangeLog: Add line-height to Text, TextInput, and StyledText.

Author: tilladam

docs/development/text-layout.md

@@ -14,6 +14,7 @@ Slint's text layout system handles the complex process of converting text string
 - **Script-aware boundaries**: Splitting text by Unicode script for font selection
 - **Line breaking**: Unicode-compliant line break algorithm
 - **Text wrapping**: Word wrap, character wrap, and no wrap modes
+- **Line height**: Natural font metrics or an explicit line height
 - **Text overflow**: Clipping and elision (ellipsis)
 - **Styled text**: Markdown parsing with formatting spans
 
@@ -111,8 +112,8 @@ pub trait TextShaper {
     /// Get glyph for a single character (e.g., ellipsis)
     fn glyph_for_char(&self, ch: char) -> Option<Glyph<Self::Length>>;
 
-    /// Calculate max lines that fit in height
-    fn max_lines(&self, max_height: Self::Length) -> usize;
+    /// Returns how many lines of `line_height` fit in `max_height`.
+    fn max_lines(&self, max_height: Self::Length, line_height: Self::Length) -> usize;
 }
 ```
 
@@ -138,6 +139,22 @@ Combined trait for fonts:
 pub trait AbstractFont: TextShaper + FontMetrics<<Self as TextShaper>::Length> {}
 ```
 
+### TextLayout
+
+`TextLayout` combines a font with paragraph-wide spacing options:
+
+```rust
+pub struct TextLayout<'a, Font: AbstractFont> {
+    pub font: &'a Font,
+    pub letter_spacing: Option<<Font as TextShaper>::Length>,
+    pub line_height: Option<<Font as TextShaper>::Length>,
+}
+```
+
+Both spacing values use the font's `Length` unit.
+`None` leaves letter spacing unchanged and uses `FontMetrics::height()` for line height.
+The `.slint` `line-height` multiplier is converted to an absolute value before this stage.
+
 ## Script Boundary Detection
 
 The `ShapeBoundaries` iterator splits text by Unicode script for optimal font selection:
@@ -273,6 +290,7 @@ pub struct TextParagraphLayout<'a, Font: AbstractFont> {
     pub wrap: TextWrap,
     pub overflow: TextOverflow,
     pub single_line: bool,
+    pub max_lines: Option<usize>,
 }
 ```
 
@@ -432,11 +450,12 @@ impl StyledText {
 ### Measuring Text
 
 ```rust
-let layout = TextLayout { font: &font, letter_spacing: None };
+let layout = TextLayout { font: &font, letter_spacing: None, line_height: None };
 let (width, height) = layout.text_size(
     "Hello World",
     Some(max_width),  // None for unconstrained
     TextWrap::WordWrap,
+    None,             // No line limit
 );
 ```
 
@@ -445,14 +464,15 @@ let (width, height) = layout.text_size(
 ```rust
 let paragraph = TextParagraphLayout {
     string: text,
-    layout: TextLayout { font: &font, letter_spacing: None },
+    layout: TextLayout { font: &font, letter_spacing: None, line_height: None },
     max_width: 200.0,
     max_height: 100.0,
     horizontal_alignment: TextHorizontalAlignment::Left,
     vertical_alignment: TextVerticalAlignment::Top,
     wrap: TextWrap::WordWrap,
     overflow: TextOverflow::Elide,
     single_line: false,
+    max_lines: None,
 };
 
 paragraph.layout_lines::<()>(
@@ -501,8 +521,8 @@ impl TextShaper for MyFont {
         // ... build glyph
     }
 
-    fn max_lines(&self, max_height: f32) -> usize {
-        (max_height / self.height()).floor() as usize
+    fn max_lines(&self, max_height: f32, line_height: f32) -> usize {
+        (max_height / line_height).floor() as usize
     }
 }
 ```

internal/backends/testing/testing_backend.rs

@@ -2,7 +2,10 @@
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
 use i_slint_core::api::PhysicalSize;
-use i_slint_core::graphics::euclid::{Point2D, Size2D};
+use i_slint_core::graphics::{
+    FontRequest,
+    euclid::{Point2D, Size2D},
+};
 use i_slint_core::item_rendering::HasFont;
 use i_slint_core::lengths::{LogicalLength, LogicalPoint, LogicalRect, LogicalSize};
 use i_slint_core::platform::PlatformError;
@@ -140,6 +143,10 @@ fn is_fixed_test_font(family: &Option<SharedString>) -> bool {
     family.as_ref().is_some_and(|f| f == FIXED_TEST_FONT)
 }
 
+fn fixed_test_font_line_height(font_request: &FontRequest, pixel_size: f32) -> f32 {
+    font_request.line_height_for_font_size(pixel_size).unwrap_or(pixel_size)
+}
+
 #[derive(Default)]
 pub struct TestingBackendOptions {
     pub mock_time: bool,
@@ -523,7 +530,8 @@ impl RendererSealed for TestingWindow {
                 .take(max_lines)
                 .fold((0, 0), |(len, count), line| (len.max(line.len()), count + 1));
             let width = max_line_len as f32 * pixel_size;
-            let height = num_lines.max(1) as f32 * pixel_size;
+            let height =
+                num_lines.max(1) as f32 * fixed_test_font_line_height(&font_request, pixel_size);
             LogicalSize::new(width, height)
         } else {
             sharedparley::text_size(self, text_item, item_rc, max_width, text_wrap, None)
@@ -568,7 +576,7 @@ impl RendererSealed for TestingWindow {
         let font_request = text_item.font_request(item_rc);
         if is_fixed_test_font(&font_request.family) {
             let pixel_size = font_request.pixel_size.map_or(10., |s| s.get());
-            LogicalSize::new(pixel_size, pixel_size)
+            LogicalSize::new(pixel_size, fixed_test_font_line_height(&font_request, pixel_size))
         } else {
             let Some(ctx) = self.slint_context() else {
                 return LogicalSize::default();
@@ -608,16 +616,13 @@ impl RendererSealed for TestingWindow {
         let font_request = text_input.font_request(item_rc);
         if is_fixed_test_font(&font_request.family) {
             let pixel_size = font_request.pixel_size.map_or(10., |s| s.get());
+            let line_height = fixed_test_font_line_height(&font_request, pixel_size);
             let text = text_input.text();
             if pos.y < 0. {
                 return 0;
             }
-            let line = (pos.y / pixel_size) as usize;
-            let offset = if line >= 1 {
-                text.split('\n').take(line - 1).map(|l| l.len() + 1).sum()
-            } else {
-                0
-            };
+            let line = (pos.y / line_height) as usize;
+            let offset: usize = text.split('\n').take(line).map(|l| l.len() + 1).sum();
             let Some(line) = text.split('\n').nth(line) else {
                 return text.len();
             };
@@ -637,12 +642,13 @@ impl RendererSealed for TestingWindow {
         let font_request = text_input.font_request(item_rc);
         if is_fixed_test_font(&font_request.family) {
             let pixel_size = font_request.pixel_size.map_or(10., |s| s.get());
+            let line_height = fixed_test_font_line_height(&font_request, pixel_size);
             let text = text_input.text();
             let line = text[..byte_offset].chars().filter(|c| *c == '\n').count();
             let column = text[..byte_offset].split('\n').nth(line).unwrap_or("").len();
             LogicalRect::new(
-                Point2D::new(column as f32 * pixel_size, line as f32 * pixel_size),
-                Size2D::new(1., pixel_size),
+                Point2D::new(column as f32 * pixel_size, line as f32 * line_height),
+                Size2D::new(1., line_height),
             )
         } else {
             sharedparley::text_input_cursor_rect_for_byte_offset(

internal/compiler/builtins.slint

@@ -658,6 +658,20 @@ component ComplexText inherits SimpleText {
     /// }
     /// ```
     in property <length> letter-spacing;
+    /// The line height as a unitless multiple of the font size or as a percentage.
+    /// Values less than or equal to zero use the natural line height.
+    /// The CSS `normal` keyword and length values aren't supported.
+    ///
+    /// ```slint "line-height: 1.5;" imageAlt="text with increased line height" width="200" height="200" needsBackground
+    /// Text {
+    ///     text: "Two lines\nof text";
+    ///     color: black;
+    ///     font-size: 30pt;
+    ///     line-height: 1.5;
+    /// }
+    /// ```
+    /// \default 0
+    in property <float> line-height;
     /// The brush used for the text outline.
     /// ```slint "stroke: darkblue;" imageAlt="text stroke" width="300" height="200" needsBackground
     /// Text {
@@ -747,6 +761,11 @@ component StyledTextItem inherits Empty {
     in property <string> default-font-family;
     /// The default font size used to render the text, when no size is specified via markup. If unset (or zero), the value falls back to the enclosing `Window`'s `default-font-size`.
     in property <length> default-font-size;
+    /// The line height as a unitless multiple of the font size or as a percentage.
+    /// Values less than or equal to zero use the natural line height.
+    /// The CSS `normal` keyword and length values aren't supported.
+    /// \default 0
+    in property <float> line-height;
     /// The horizontal alignment of the text.
     in property <TextHorizontalAlignment> horizontal-alignment;
     /// The color used for rendering links in the text.
@@ -1737,6 +1756,11 @@ export component TextInput {
     /// The letter spacing allows changing the spacing between the glyphs. A positive value increases the spacing and a negative value decreases the distance.
     /// \default 0
     in property <length> letter-spacing;
+    /// The line height as a unitless multiple of the font size or as a percentage.
+    /// Values less than or equal to zero use the natural line height.
+    /// The CSS `normal` keyword and length values aren't supported.
+    /// \default 0
+    in property <float> line-height;
     in property <length> width;
     in property <length> height;
     /// The height of the page used to compute how much to scroll when the user presses page up or page down.

internal/compiler/passes/binding_analysis.rs

@@ -953,6 +953,7 @@ fn visit_implicit_layout_info_dependencies(
             vis(&NamedReference::new(item, SmolStr::new_static("font-size")).into(), N);
             vis(&NamedReference::new(item, SmolStr::new_static("font-weight")).into(), N);
             vis(&NamedReference::new(item, SmolStr::new_static("letter-spacing")).into(), N);
+            vis(&NamedReference::new(item, SmolStr::new_static("line-height")).into(), N);
             vis(&NamedReference::new(item, SmolStr::new_static("wrap")).into(), N);
             let wrap_set = item.borrow().is_binding_set("wrap", false)
                 || item

internal/core/graphics.rs

@@ -99,10 +99,21 @@ pub struct FontRequest {
     /// The additional spacing (or shrinking if negative) between glyphs. This is usually not submitted to
     /// the font-subsystem but collected here for API convenience
     pub letter_spacing: Option<LogicalLength>,
+    /// The line height as a multiple of the font size.
+    /// `None` uses the font's natural line height.
+    pub line_height: Option<f32>,
     /// Whether to select an italic face of the font family.
     pub italic: bool,
 }
 
+impl FontRequest {
+    /// Returns the configured line height in the same unit as `font_size`,
+    /// or `None` when the font's natural line height applies.
+    pub fn line_height_for_font_size(&self, font_size: f32) -> Option<f32> {
+        self.line_height.map(|line_height| font_size * line_height)
+    }
+}
+
 #[cfg(feature = "shared-fontique")]
 impl FontRequest {
     /// Attempts to query the fontique font collection for a matching font.

internal/core/item_rendering.rs

@@ -394,6 +394,7 @@ impl HasFont for (SharedString, Brush) {
             0,
             LogicalLength::default(),
             LogicalLength::default(),
+            0.0,
             false,
         )
     }

internal/core/items.rs

@@ -1453,6 +1453,7 @@ impl WindowItem {
         local_font_weight: i32,
         local_font_size: LogicalLength,
         local_letter_spacing: LogicalLength,
+        local_line_height: f32,
         local_italic: bool,
     ) -> FontRequest {
         let Some(window_item_rc) = next_window_item(self_rc) else {
@@ -1492,6 +1493,8 @@ impl WindowItem {
                 }
             },
             letter_spacing: Some(local_letter_spacing),
+            line_height: (local_line_height.is_finite() && local_line_height > 0.0)
+                .then_some(local_line_height),
             italic: local_italic,
         }
     }

internal/core/items/text.rs

@@ -59,6 +59,7 @@ pub struct ComplexText {
     pub wrap: Property<TextWrap>,
     pub overflow: Property<TextOverflow>,
     pub letter_spacing: Property<LogicalLength>,
+    pub line_height: Property<f32>,
     pub stroke: Property<Brush>,
     pub stroke_width: Property<LogicalLength>,
     pub stroke_style: Property<TextStrokeStyle>,
@@ -173,6 +174,7 @@ impl HasFont for ComplexText {
             self.font_weight(),
             self.font_size(),
             self.letter_spacing(),
+            self.line_height(),
             self.font_italic(),
         )
     }
@@ -246,6 +248,7 @@ pub struct StyledTextItem {
     pub default_color: Property<Brush>,
     pub default_font_size: Property<LogicalLength>,
     pub default_font_family: Property<SharedString>,
+    pub line_height: Property<f32>,
     pub horizontal_alignment: Property<TextHorizontalAlignment>,
     pub vertical_alignment: Property<TextVerticalAlignment>,
     pub max_lines: Property<i32>,
@@ -401,6 +404,7 @@ impl HasFont for StyledTextItem {
             Default::default(),
             self.default_font_size(),
             Default::default(),
+            self.line_height(),
             Default::default(),
         )
     }
@@ -589,6 +593,7 @@ impl HasFont for SimpleText {
             self.font_weight(),
             self.font_size(),
             LogicalLength::default(),
+            0.0,
             false,
         )
     }
@@ -772,6 +777,7 @@ pub struct TextInput {
     pub input_type: Property<InputType>,
     pub input_method_hints: Property<InputMethodHints>,
     pub letter_spacing: Property<LogicalLength>,
+    pub line_height: Property<f32>,
     pub width: Property<LogicalLength>,
     pub height: Property<LogicalLength>,
     pub cursor_position_byte_offset: Property<i32>,
@@ -1322,6 +1328,7 @@ impl HasFont for TextInput {
             self.font_weight(),
             self.font_size(),
             self.letter_spacing(),
+            self.line_height(),
             self.font_italic(),
         )
     }