gui: box model style layout/render for fancy tab bar

This commit adds a CSS box model inspired element / layout
facility, and replaces the hand implemented fancy tab bar
element render.

This makes the code for fancy tab bar much easier to read
and update.

The right status area now expands to the full height of the
tab bar area, and uses a line height of 2.0, which makes
it line up nicely in the tab bar.

config/src/units.rs

@@ -80,7 +80,7 @@ impl<'de> serde::de::Visitor<'de> for DefaultUnit {
     }
 }
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub enum Dimension {
     /// A value expressed in points, where 72 points == 1 inch.
     Points(f32),

wezterm-font/src/lib.rs

@@ -52,6 +52,19 @@ pub struct LoadedFont {
     text_style: TextStyle,
 }
 
+impl std::fmt::Debug for LoadedFont {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
+        fmt.debug_struct("LoadedFont")
+            .field("handles", &self.handles)
+            .field("metrics", &self.metrics)
+            .field("font_size", &self.font_size)
+            .field("dpi", &self.dpi)
+            .field("pending_fallback", &self.pending_fallback)
+            .field("text_style", &self.text_style)
+            .finish()
+    }
+}
+
 impl LoadedFont {
     pub fn metrics(&self) -> FontMetrics {
         self.metrics

wezterm-gui/src/customglyph.rs

@@ -236,11 +236,12 @@ impl BlockKey {
     }
 
     pub fn from_str(s: &str) -> Option<Self> {
-        let chars = s.chars().collect::<Vec<char>>();
-        if chars.len() == 1 {
-            Self::from_char(chars[0])
-        } else {
+        let mut chars = s.chars();
+        let first_char = chars.next()?;
+        if chars.next().is_some() {
             None
+        } else {
+            Self::from_char(first_char)
         }
     }
 

wezterm-gui/src/termwindow/box_model.rs

@@ -0,0 +1,948 @@
+#![allow(dead_code)]
+use crate::color::LinearRgba;
+use crate::customglyph::{BlockKey, Poly};
+use crate::glyphcache::CachedGlyph;
+use crate::termwindow::render::rgbcolor_to_window_color;
+use crate::termwindow::{
+    MappedQuads, RenderState, SrgbTexture2d, TermWindowNotif, UIItem, UIItemType,
+};
+use crate::utilsprites::RenderMetrics;
+use ::window::{RectF, WindowOps};
+use anyhow::anyhow;
+use config::{Dimension, DimensionContext};
+use std::ops::Sub;
+use std::rc::Rc;
+use termwiz::cell::{grapheme_column_width, Presentation};
+use termwiz::surface::Line;
+use unicode_segmentation::UnicodeSegmentation;
+use wezterm_font::units::PixelUnit;
+use wezterm_font::LoadedFont;
+use wezterm_term::color::{ColorAttribute, ColorPalette};
+use window::bitmaps::atlas::Sprite;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DisplayType {
+    Block,
+    Inline,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Float {
+    None,
+    Right,
+}
+
+impl Default for Float {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct PixelDimension {
+    pub left: f32,
+    pub top: f32,
+    pub right: f32,
+    pub bottom: f32,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct PixelSizedPoly {
+    pub poly: &'static [Poly],
+    pub width: f32,
+    pub height: f32,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct SizedPoly {
+    pub poly: &'static [Poly],
+    pub width: Dimension,
+    pub height: Dimension,
+}
+
+impl SizedPoly {
+    pub fn to_pixels(&self, context: &LayoutContext) -> PixelSizedPoly {
+        PixelSizedPoly {
+            poly: self.poly,
+            width: self.width.evaluate_as_pixels(context.width),
+            height: self.height.evaluate_as_pixels(context.height),
+        }
+    }
+
+    pub fn none() -> Self {
+        Self {
+            poly: &[],
+            width: Dimension::default(),
+            height: Dimension::default(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct PixelCorners {
+    pub top_left: PixelSizedPoly,
+    pub top_right: PixelSizedPoly,
+    pub bottom_left: PixelSizedPoly,
+    pub bottom_right: PixelSizedPoly,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct Corners {
+    pub top_left: SizedPoly,
+    pub top_right: SizedPoly,
+    pub bottom_left: SizedPoly,
+    pub bottom_right: SizedPoly,
+}
+
+impl Corners {
+    pub fn to_pixels(&self, context: &LayoutContext) -> PixelCorners {
+        PixelCorners {
+            top_left: self.top_left.to_pixels(context),
+            top_right: self.top_right.to_pixels(context),
+            bottom_left: self.bottom_left.to_pixels(context),
+            bottom_right: self.bottom_left.to_pixels(context),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct BoxDimension {
+    pub left: Dimension,
+    pub top: Dimension,
+    pub right: Dimension,
+    pub bottom: Dimension,
+}
+
+impl BoxDimension {
+    pub const fn new(dim: Dimension) -> Self {
+        Self {
+            left: dim,
+            top: dim,
+            right: dim,
+            bottom: dim,
+        }
+    }
+
+    pub fn to_pixels(&self, context: &LayoutContext) -> PixelDimension {
+        PixelDimension {
+            left: self.left.evaluate_as_pixels(context.width),
+            top: self.top.evaluate_as_pixels(context.height),
+            right: self.right.evaluate_as_pixels(context.width),
+            bottom: self.bottom.evaluate_as_pixels(context.height),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum InheritableColor {
+    Inherited,
+    Color(LinearRgba),
+}
+
+impl Default for InheritableColor {
+    fn default() -> Self {
+        Self::Inherited
+    }
+}
+
+impl From<LinearRgba> for InheritableColor {
+    fn from(color: LinearRgba) -> InheritableColor {
+        InheritableColor::Color(color)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct BorderColor {
+    pub left: LinearRgba,
+    pub top: LinearRgba,
+    pub right: LinearRgba,
+    pub bottom: LinearRgba,
+}
+
+impl BorderColor {
+    pub const fn new(color: LinearRgba) -> Self {
+        Self {
+            left: color,
+            top: color,
+            right: color,
+            bottom: color,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct ElementColors {
+    pub border: BorderColor,
+    pub bg: InheritableColor,
+    pub text: InheritableColor,
+}
+
+impl ElementColors {
+    pub fn resolve_bg(&self, inherited_colors: Option<&ElementColors>) -> LinearRgba {
+        match self.bg {
+            InheritableColor::Inherited => match inherited_colors {
+                Some(colors) => colors.resolve_bg(None),
+                None => LinearRgba::TRANSPARENT,
+            },
+            InheritableColor::Color(color) => color,
+        }
+    }
+
+    pub fn resolve_text(&self, inherited_colors: Option<&ElementColors>) -> LinearRgba {
+        match self.text {
+            InheritableColor::Inherited => match inherited_colors {
+                Some(colors) => colors.resolve_text(None),
+                None => LinearRgba::TRANSPARENT,
+            },
+            InheritableColor::Color(color) => color,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Element {
+    pub item_type: Option<UIItemType>,
+    pub zindex: i8,
+    pub display: DisplayType,
+    pub float: Float,
+    pub padding: BoxDimension,
+    pub margin: BoxDimension,
+    pub border: BoxDimension,
+    pub border_corners: Option<Corners>,
+    pub colors: ElementColors,
+    pub hover_colors: Option<ElementColors>,
+    pub font: Rc<LoadedFont>,
+    pub content: ElementContent,
+    pub presentation: Option<Presentation>,
+    pub line_height: Option<f64>,
+}
+
+impl Element {
+    pub fn new(font: &Rc<LoadedFont>, content: ElementContent) -> Self {
+        Self {
+            item_type: None,
+            zindex: 0,
+            display: DisplayType::Inline,
+            float: Float::None,
+            padding: BoxDimension::default(),
+            margin: BoxDimension::default(),
+            border: BoxDimension::default(),
+            border_corners: None,
+            colors: ElementColors::default(),
+            hover_colors: None,
+            font: Rc::clone(font),
+            content,
+            presentation: None,
+            line_height: None,
+        }
+    }
+
+    pub fn with_line(font: &Rc<LoadedFont>, line: &Line, palette: &ColorPalette) -> Self {
+        let mut content = vec![];
+
+        for cluster in line.cluster() {
+            let child =
+                Element::new(font, ElementContent::Text(cluster.text)).colors(ElementColors {
+                    border: BorderColor::default(),
+                    bg: if cluster.attrs.background() == ColorAttribute::Default {
+                        InheritableColor::Inherited
+                    } else {
+                        rgbcolor_to_window_color(palette.resolve_bg(cluster.attrs.background()))
+                            .into()
+                    },
+                    text: if cluster.attrs.foreground() == ColorAttribute::Default {
+                        InheritableColor::Inherited
+                    } else {
+                        rgbcolor_to_window_color(palette.resolve_fg(cluster.attrs.foreground()))
+                            .into()
+                    },
+                });
+
+            content.push(child);
+        }
+
+        Self::new(font, ElementContent::Children(content))
+    }
+
+    pub fn item_type(mut self, item_type: UIItemType) -> Self {
+        self.item_type.replace(item_type);
+        self
+    }
+
+    pub fn display(mut self, display: DisplayType) -> Self {
+        self.display = display;
+        self
+    }
+
+    pub fn float(mut self, float: Float) -> Self {
+        self.float = float;
+        self
+    }
+
+    pub fn colors(mut self, colors: ElementColors) -> Self {
+        self.colors = colors;
+        self
+    }
+
+    pub fn hover_colors(mut self, colors: Option<ElementColors>) -> Self {
+        self.hover_colors = colors;
+        self
+    }
+
+    pub fn line_height(mut self, line_height: Option<f64>) -> Self {
+        self.line_height = line_height;
+        self
+    }
+
+    pub fn zindex(mut self, zindex: i8) -> Self {
+        self.zindex = zindex;
+        self
+    }
+
+    pub fn padding(mut self, padding: BoxDimension) -> Self {
+        self.padding = padding;
+        self
+    }
+
+    pub fn border(mut self, border: BoxDimension) -> Self {
+        self.border = border;
+        self
+    }
+
+    pub fn border_corners(mut self, corners: Option<Corners>) -> Self {
+        self.border_corners = corners;
+        self
+    }
+
+    pub fn margin(mut self, margin: BoxDimension) -> Self {
+        self.margin = margin;
+        self
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum ElementContent {
+    Text(String),
+    Children(Vec<Element>),
+}
+
+pub struct LayoutContext<'a> {
+    pub width: DimensionContext,
+    pub height: DimensionContext,
+    pub bounds: RectF,
+    pub metrics: &'a RenderMetrics,
+    pub gl_state: &'a RenderState,
+}
+
+#[derive(Debug, Clone)]
+pub struct ComputedElement {
+    pub item_type: Option<UIItemType>,
+    pub zindex: i8,
+    /// The outer bounds of the element box (its margin)
+    pub bounds: RectF,
+    /// The outer bounds of the area enclosed by its border
+    pub border_rect: RectF,
+    pub border: PixelDimension,
+    pub border_corners: Option<PixelCorners>,
+    pub colors: ElementColors,
+    pub hover_colors: Option<ElementColors>,
+    /// The outer bounds of the area enclosed by the padding
+    pub padding: RectF,
+    /// The outer bounds of the content
+    pub content_rect: RectF,
+    pub baseline: f32,
+
+    pub content: ComputedElementContent,
+}
+
+impl ComputedElement {
+    fn translate(&mut self, delta: euclid::Vector2D<f32, PixelUnit>) {
+        self.bounds = self.bounds.translate(delta);
+        self.border_rect = self.border_rect.translate(delta);
+        self.padding = self.padding.translate(delta);
+        self.content_rect = self.content_rect.translate(delta);
+
+        match &mut self.content {
+            ComputedElementContent::Children(kids) => {
+                for kid in kids {
+                    kid.translate(delta)
+                }
+            }
+            ComputedElementContent::Text(_) => {}
+        }
+    }
+
+    pub fn ui_items(&self) -> Vec<UIItem> {
+        let mut items = vec![];
+        self.ui_item_impl(&mut items);
+        items
+    }
+
+    fn ui_item_impl(&self, items: &mut Vec<UIItem>) {
+        if let Some(item_type) = &self.item_type {
+            items.push(UIItem {
+                x: self.bounds.min_x().max(0.) as usize,
+                y: self.bounds.min_y().max(0.) as usize,
+                width: self.bounds.width().max(0.) as usize,
+                height: self.bounds.height().max(0.) as usize,
+                item_type: item_type.clone(),
+            });
+        }
+
+        match &self.content {
+            ComputedElementContent::Text(_) => {}
+            ComputedElementContent::Children(kids) => {
+                for kid in kids {
+                    kid.ui_item_impl(items);
+                }
+            }
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum ComputedElementContent {
+    Text(Vec<ElementCell>),
+    Children(Vec<ComputedElement>),
+}
+
+#[derive(Debug, Clone)]
+pub enum ElementCell {
+    Sprite(Sprite<SrgbTexture2d>),
+    Glyph(Rc<CachedGlyph<SrgbTexture2d>>),
+}
+
+impl super::TermWindow {
+    pub fn compute_element<'a>(
+        &self,
+        context: &LayoutContext,
+        element: &Element,
+    ) -> anyhow::Result<ComputedElement> {
+        let local_metrics;
+        let local_context;
+        let context = if let Some(line_height) = element.line_height {
+            local_metrics = context.metrics.scale_line_height(line_height);
+            local_context = LayoutContext {
+                height: DimensionContext {
+                    dpi: context.height.dpi,
+                    pixel_max: context.height.pixel_max,
+                    pixel_cell: context.height.pixel_cell * line_height as f32,
+                },
+                width: context.width,
+                bounds: context.bounds,
+                gl_state: context.gl_state,
+                metrics: &local_metrics,
+            };
+            &local_context
+        } else {
+            context
+        };
+        let padding = element.padding.to_pixels(context);
+        let margin = element.margin.to_pixels(context);
+        let border = element.border.to_pixels(context);
+        let border_corners = element
+            .border_corners
+            .as_ref()
+            .map(|c| c.to_pixels(context));
+        let style = element.font.style();
+        let baseline = context.height.pixel_cell + context.metrics.descender.get() as f32;
+
+        match &element.content {
+            ElementContent::Text(s) => {
+                let window = self.window.as_ref().unwrap().clone();
+                let infos = element.font.shape(
+                    &s,
+                    move || window.notify(TermWindowNotif::InvalidateShapeCache),
+                    BlockKey::filter_out_synthetic,
+                    element.presentation,
+                )?;
+                let mut computed_cells = vec![];
+                let mut glyph_cache = context.gl_state.glyph_cache.borrow_mut();
+                let mut pixel_width = 0.0;
+                let mut min_y = 0.0f32;
+
+                for info in infos {
+                    let cell_start = &s[info.cluster as usize..];
+                    let mut iter = cell_start.graphemes(true).peekable();
+                    let grapheme = iter
+                        .next()
+                        .ok_or_else(|| anyhow!("info.cluster didn't map into string"))?;
+                    if let Some(key) = BlockKey::from_str(grapheme) {
+                        pixel_width += context.width.pixel_cell;
+                        let sprite = glyph_cache.cached_block(key, context.metrics)?;
+                        computed_cells.push(ElementCell::Sprite(sprite));
+                    } else {
+                        let next_grapheme: Option<&str> = iter.peek().map(|s| *s);
+                        let followed_by_space = next_grapheme == Some(" ");
+                        let num_cells = grapheme_column_width(grapheme, None);
+                        let glyph = glyph_cache.cached_glyph(
+                            &info,
+                            style,
+                            followed_by_space,
+                            &element.font,
+                            context.metrics,
+                            num_cells as u8,
+                        )?;
+
+                        min_y =
+                            min_y.min(baseline - (glyph.y_offset + glyph.bearing_y).get() as f32);
+
+                        pixel_width += glyph.x_advance.get() as f32;
+                        computed_cells.push(ElementCell::Glyph(glyph));
+                    }
+                }
+
+                let content_rect = euclid::rect(0., 0., pixel_width, context.height.pixel_cell);
+
+                let padding = euclid::rect(
+                    content_rect.min_x() - padding.left,
+                    content_rect.min_y() - padding.top,
+                    content_rect.width() + padding.left + padding.right,
+                    content_rect.height() + padding.top + padding.bottom,
+                );
+
+                let border_rect = euclid::rect(
+                    padding.min_x() - border.left,
+                    padding.min_y() - border.top,
+                    padding.width() + border.left + border.right,
+                    padding.height() + border.top + border.bottom,
+                );
+
+                let bounds = euclid::rect(
+                    border_rect.min_x() - margin.left,
+                    border_rect.min_y() - margin.top,
+                    border_rect.width() + margin.left + margin.right,
+                    border_rect.height() + margin.top + margin.bottom,
+                );
+
+                let translate = euclid::vec2(
+                    context.bounds.min_x() - bounds.min_x(),
+                    context.bounds.min_y() - bounds.min_y(),
+                );
+
+                Ok(ComputedElement {
+                    item_type: element.item_type.clone(),
+                    zindex: element.zindex,
+                    baseline,
+                    border,
+                    border_corners,
+                    colors: element.colors,
+                    hover_colors: element.hover_colors,
+                    bounds: bounds.translate(translate),
+                    border_rect: border_rect.translate(translate),
+                    padding: padding.translate(translate),
+                    content_rect: content_rect.translate(translate),
+                    content: ComputedElementContent::Text(computed_cells),
+                })
+            }
+            ElementContent::Children(kids) => {
+                let mut pixel_width: f32 = 0.;
+                let mut pixel_height: f32 = 0.;
+                let mut computed_kids = vec![];
+                let mut max_x: f32 = 0.;
+
+                for child in kids {
+                    let mut kid = self.compute_element(
+                        &LayoutContext {
+                            bounds: match child.float {
+                                Float::None => euclid::rect(
+                                    pixel_width,
+                                    context.bounds.min_y(),
+                                    context.bounds.max_x() - (context.bounds.min_x() + pixel_width),
+                                    context.bounds.height(),
+                                ),
+                                Float::Right => euclid::rect(
+                                    0.,
+                                    context.bounds.min_y(),
+                                    context.bounds.width(),
+                                    context.bounds.height(),
+                                ),
+                            },
+                            gl_state: context.gl_state,
+                            height: context.height,
+                            metrics: context.metrics,
+                            width: context.width,
+                        },
+                        child,
+                    )?;
+                    match child.float {
+                        Float::Right => {
+                            let padded_max_x = context
+                                .bounds
+                                .max_x()
+                                .sub(padding.left)
+                                .sub(padding.right)
+                                .sub(margin.left)
+                                .sub(margin.right)
+                                .sub(border.left)
+                                .sub(border.right)
+                                .max(0.);
+
+                            kid.translate(euclid::vec2(padded_max_x - kid.bounds.width(), 0.));
+                            max_x = max_x.max(padded_max_x);
+                        }
+                        Float::None => {
+                            pixel_width += kid.bounds.width();
+                            max_x = max_x.max(pixel_width);
+                        }
+                    }
+                    pixel_height = pixel_height.max(kid.bounds.height());
+
+                    computed_kids.push(kid);
+                }
+
+                computed_kids.sort_by(|a, b| a.zindex.cmp(&b.zindex));
+
+                let content_rect = euclid::rect(0., 0., max_x, pixel_height);
+
+                let padding = euclid::rect(
+                    content_rect.min_x() - padding.left,
+                    content_rect.min_y() - padding.top,
+                    content_rect.width() + padding.left + padding.right,
+                    content_rect.height() + padding.top + padding.bottom,
+                );
+
+                let border_rect = euclid::rect(
+                    padding.min_x() - border.left,
+                    padding.min_y() - border.top,
+                    padding.width() + border.left + border.right,
+                    padding.height() + border.top + border.bottom,
+                );
+
+                let bounds = euclid::rect(
+                    border_rect.min_x() - margin.left,
+                    border_rect.min_y() - margin.top,
+                    border_rect.width() + margin.left + margin.right,
+                    border_rect.height() + margin.top + margin.bottom,
+                );
+
+                let translate = euclid::vec2(
+                    context.bounds.min_x() - bounds.min_x(),
+                    context.bounds.min_y() - bounds.min_y(),
+                );
+
+                for kid in &mut computed_kids {
+                    kid.translate(translate);
+                }
+
+                Ok(ComputedElement {
+                    item_type: element.item_type.clone(),
+                    zindex: element.zindex,
+                    baseline,
+                    border,
+                    border_corners,
+                    colors: element.colors,
+                    hover_colors: element.hover_colors,
+                    bounds: bounds.translate(translate),
+                    border_rect: border_rect.translate(translate),
+                    padding: padding.translate(translate),
+                    content_rect: content_rect.translate(translate),
+                    content: ComputedElementContent::Children(computed_kids),
+                })
+            }
+        }
+    }
+
+    pub fn render_element<'a>(
+        &self,
+        element: &ComputedElement,
+        layer: &'a mut MappedQuads,
+        inherited_colors: Option<&ElementColors>,
+    ) -> anyhow::Result<()> {
+        let colors = match &element.hover_colors {
+            Some(hc) => {
+                let hovering = match &self.current_mouse_event {
+                    Some(event) => {
+                        let mouse_x = event.coords.x as f32;
+                        let mouse_y = event.coords.y as f32;
+                        mouse_x >= element.bounds.min_x()
+                            && mouse_x <= element.bounds.max_x()
+                            && mouse_y >= element.bounds.min_y()
+                            && mouse_y <= element.bounds.max_y()
+                    }
+                    None => false,
+                };
+                if hovering {
+                    hc
+                } else {
+                    &element.colors
+                }
+            }
+            None => &element.colors,
+        };
+
+        self.render_element_background(element, colors, layer, inherited_colors)?;
+        let left = self.dimensions.pixel_width as f32 / -2.0;
+        let top = self.dimensions.pixel_height as f32 / -2.0;
+        match &element.content {
+            ComputedElementContent::Text(cells) => {
+                let mut pos_x = element.content_rect.min_x() as f32;
+                for cell in cells {
+                    match cell {
+                        ElementCell::Sprite(sprite) => {
+                            let mut quad = layer.allocate()?;
+                            let width = sprite.coords.width();
+                            let height = sprite.coords.height();
+                            let pos_y = top + element.content_rect.min_y() as f32;
+                            quad.set_position(
+                                pos_x + left,
+                                pos_y,
+                                pos_x + left + width as f32,
+                                pos_y + height as f32,
+                            );
+                            quad.set_fg_color(colors.resolve_text(inherited_colors));
+                            quad.set_texture(sprite.texture_coords());
+                            quad.set_hsv(None);
+                            pos_x += width as f32;
+                        }
+                        ElementCell::Glyph(glyph) => {
+                            if let Some(texture) = glyph.texture.as_ref() {
+                                let mut quad = layer.allocate()?;
+                                let pos_y = element.content_rect.min_y() as f32 + top
+                                    - (glyph.y_offset + glyph.bearing_y).get() as f32
+                                    + element.baseline;
+
+                                let width = texture.coords.size.width as f32 * glyph.scale as f32;
+                                let height = texture.coords.size.height as f32 * glyph.scale as f32;
+
+                                quad.set_position(
+                                    pos_x + left,
+                                    pos_y,
+                                    pos_x + left + width,
+                                    pos_y + height,
+                                );
+                                quad.set_fg_color(colors.resolve_text(inherited_colors));
+                                quad.set_texture(texture.texture_coords());
+                                quad.set_has_color(glyph.has_color);
+                                quad.set_hsv(None);
+                            }
+                            pos_x += glyph.x_advance.get() as f32;
+                        }
+                    }
+                }
+            }
+            ComputedElementContent::Children(kids) => {
+                for kid in kids {
+                    self.render_element(kid, layer, Some(colors))?;
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    fn render_element_background<'a>(
+        &self,
+        element: &ComputedElement,
+        colors: &ElementColors,
+        layer: &'a mut MappedQuads,
+        inherited_colors: Option<&ElementColors>,
+    ) -> anyhow::Result<()> {
+        let mut top_left_width = 0;
+        let mut top_left_height = 0;
+        let mut top_right_width = 0;
+        let mut top_right_height = 0;
+
+        let mut bottom_left_width = 0;
+        let mut bottom_left_height = 0;
+        let mut bottom_right_width = 0;
+        let mut bottom_right_height = 0;
+
+        if let Some(c) = &element.border_corners {
+            top_left_width = c.top_left.width as isize;
+            top_left_height = c.top_left.height as isize;
+            top_right_width = c.top_right.width as isize;
+            top_right_height = c.top_right.height as isize;
+
+            bottom_left_width = c.bottom_left.width as isize;
+            bottom_left_height = c.bottom_left.height as isize;
+            bottom_right_width = c.bottom_right.width as isize;
+            bottom_right_height = c.bottom_right.height as isize;
+
+            let underline_height = 1;
+            if top_left_width > 0 && top_left_height > 0 {
+                self.poly_quad(
+                    layer,
+                    element.border_rect.origin,
+                    c.top_left.poly,
+                    underline_height,
+                    euclid::size2(top_left_width, top_left_height),
+                    colors.border.top,
+                )?;
+            }
+            if top_right_width > 0 && top_right_height > 0 {
+                self.poly_quad(
+                    layer,
+                    euclid::point2(
+                        element.border_rect.max_x() - top_right_width as f32,
+                        element.border_rect.min_y(),
+                    ),
+                    c.top_right.poly,
+                    underline_height,
+                    euclid::size2(top_right_width, top_right_height),
+                    colors.border.top,
+                )?;
+            }
+            if bottom_left_width > 0 && bottom_left_height > 0 {
+                self.poly_quad(
+                    layer,
+                    euclid::point2(
+                        element.border_rect.min_x(),
+                        element.border_rect.max_y() - bottom_left_height as f32,
+                    ),
+                    c.bottom_left.poly,
+                    underline_height,
+                    euclid::size2(bottom_left_width, bottom_left_height),
+                    colors.border.bottom,
+                )?;
+            }
+            if bottom_right_width > 0 && bottom_right_height > 0 {
+                self.poly_quad(
+                    layer,
+                    euclid::point2(
+                        element.border_rect.max_x() - bottom_right_width as f32,
+                        element.border_rect.max_y() - bottom_right_height as f32,
+                    ),
+                    c.bottom_right.poly,
+                    underline_height,
+                    euclid::size2(bottom_right_width, bottom_right_height),
+                    colors.border.bottom,
+                )?;
+            }
+
+            // Filling the background is more complex because we can't
+            // simply fill the padding rect--we'd clobber the corner
+            // graphics.
+            // Instead, we consider the element as consisting of:
+            //
+            //   TL T TR
+            //   L  C  R
+            //   BL B BR
+            //
+            // We already rendered the corner pieces, so now we need
+            // to do the rest
+
+            // The `T` piece
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x() + top_left_width as f32,
+                    element.border_rect.min_y(),
+                    element.border_rect.width() - (top_left_width + top_right_width) as f32,
+                    top_left_height.max(top_right_height) as f32,
+                ),
+                colors.resolve_bg(inherited_colors),
+            )?;
+
+            // The `B` piece
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x() + bottom_left_width as f32,
+                    element.border_rect.max_y()
+                        - bottom_left_height.max(bottom_right_height) as f32,
+                    element.border_rect.width() - (bottom_left_width + bottom_right_width) as f32,
+                    bottom_left_height.max(bottom_right_height) as f32,
+                ),
+                colors.resolve_bg(inherited_colors),
+            )?;
+
+            // The `L` piece
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x(),
+                    element.border_rect.min_y() + top_left_height as f32,
+                    top_left_width.max(bottom_left_width) as f32,
+                    element.border_rect.height() - (top_left_height + bottom_left_height) as f32,
+                ),
+                colors.resolve_bg(inherited_colors),
+            )?;
+
+            // The `R` piece
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.max_x() - top_right_width as f32,
+                    element.border_rect.min_y() + top_right_height as f32,
+                    top_right_width.max(bottom_right_width) as f32,
+                    element.border_rect.height() - (top_right_height + bottom_right_height) as f32,
+                ),
+                colors.resolve_bg(inherited_colors),
+            )?;
+
+            // The `C` piece
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x() + top_left_width as f32,
+                    element.border_rect.min_y() + top_right_height.min(top_left_height) as f32,
+                    element.border_rect.width() - (top_left_width + top_right_width) as f32,
+                    element.border_rect.height()
+                        - (top_right_height.min(top_left_height)
+                            + bottom_right_height.min(bottom_left_height))
+                            as f32,
+                ),
+                colors.resolve_bg(inherited_colors),
+            )?;
+        } else if colors.bg != InheritableColor::Color(LinearRgba::TRANSPARENT) {
+            self.filled_rectangle(layer, element.padding, colors.resolve_bg(inherited_colors))?;
+        }
+
+        if element.border_rect == element.padding {
+            // There's no border to be drawn
+            return Ok(());
+        }
+
+        if element.border.top > 0. && colors.border.top != LinearRgba::TRANSPARENT {
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x() + top_left_width as f32,
+                    element.border_rect.min_y(),
+                    element.border_rect.width() - (top_left_width + top_right_width) as f32,
+                    element.border.top,
+                ),
+                colors.border.top,
+            )?;
+        }
+        if element.border.bottom > 0. && colors.border.bottom != LinearRgba::TRANSPARENT {
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x() + bottom_left_width as f32,
+                    element.border_rect.max_y() - element.border.bottom,
+                    element.border_rect.width() - (bottom_left_width + bottom_right_width) as f32,
+                    element.border.bottom,
+                ),
+                colors.border.bottom,
+            )?;
+        }
+        if element.border.left > 0. && colors.border.left != LinearRgba::TRANSPARENT {
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.min_x(),
+                    element.border_rect.min_y() + top_left_height as f32,
+                    element.border.left,
+                    element.border_rect.height() - (top_left_height + bottom_left_height) as f32,
+                ),
+                colors.border.left,
+            )?;
+        }
+        if element.border.right > 0. && colors.border.right != LinearRgba::TRANSPARENT {
+            self.filled_rectangle(
+                layer,
+                euclid::rect(
+                    element.border_rect.max_x() - element.border.right,
+                    element.border_rect.min_y() + top_right_height as f32,
+                    element.border.left,
+                    element.border_rect.height() - (top_right_height + bottom_right_height) as f32,
+                ),
+                colors.border.right,
+            )?;
+        }
+
+        Ok(())
+    }
+}

wezterm-gui/src/termwindow/mod.rs

@@ -51,6 +51,7 @@ use wezterm_term::color::ColorPalette;
 use wezterm_term::input::LastMouseClick;
 use wezterm_term::{Alert, SemanticZone, StableRowIndex, TerminalConfiguration};
 
+pub mod box_model;
 pub mod clipboard;
 mod keyevent;
 mod mouseevent;

wezterm-gui/src/utilsprites.rs

@@ -43,6 +43,24 @@ impl RenderMetrics {
         }
     }
 
+    pub fn scale_line_height(&self, line_height: f64) -> Self {
+        let size = euclid::size2(
+            self.cell_size.width,
+            (self.cell_size.height as f64 * line_height) as isize,
+        );
+
+        let adjust = (((self.descender_row as f64 * line_height) - self.descender_row as f64) / 2.0)
+            as isize;
+        Self {
+            descender: self.descender - PixelLength::new(adjust as f64),
+            descender_row: self.descender_row - adjust,
+            descender_plus_two: self.descender_plus_two - adjust,
+            underline_height: self.underline_height,
+            strike_row: self.strike_row,
+            cell_size: size,
+        }
+    }
+
     pub fn new(fonts: &Rc<FontConfiguration>) -> anyhow::Result<Self> {
         let metrics = fonts
             .default_font_metrics()

wezterm-input-types/src/lib.rs

@@ -5,6 +5,7 @@ use std::convert::TryFrom;
 pub struct PixelUnit;
 pub struct ScreenPixelUnit;
 pub type Point = euclid::Point2D<isize, PixelUnit>;
+pub type PointF = euclid::Point2D<f32, PixelUnit>;
 pub type ScreenPoint = euclid::Point2D<isize, ScreenPixelUnit>;
 
 /// Which key is pressed.  Not all of these are probable to appear

window/src/lib.rs

@@ -51,7 +51,9 @@ pub struct Dimensions {
 }
 
 pub type Rect = euclid::Rect<isize, PixelUnit>;
+pub type RectF = euclid::Rect<f32, PixelUnit>;
 pub type Size = euclid::Size2D<isize, PixelUnit>;
+pub type SizeF = euclid::Size2D<f32, PixelUnit>;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum MouseCursor {