Support different kinds of underline rendering

Adds four new  modifiers that can be used in themes:

- undercurled
- underdashed
- underdotted
- double-underline

book/src/themes.md

@@ -13,10 +13,10 @@ The default theme.toml can be found [here](https://github.com/helix-editor/helix
 Each line in the theme file is specified as below:
 
 ```toml
-key = { fg = "#ffffff", bg = "#000000", modifiers = ["bold", "italic"] }
+key = { fg = "#ffffff", bg = "#000000", underline = "#ff0000", modifiers = ["bold", "italic", "undercurled"] }
 ```
 
-where `key` represents what you want to style, `fg` specifies the foreground color, `bg` the background color, and `modifiers` is a list of style modifiers. `bg` and `modifiers` can be omitted to defer to the defaults.
+where `key` represents what you want to style, `fg` specifies the foreground color, `bg` the background color, `underline` the underline color (only meaningful if an underline modifier is enabled), and `modifiers` is a list of style modifiers. `bg`, `underline` and `modifiers` can be omitted to defer to the defaults.
 
 To specify only the foreground color:
 
@@ -77,17 +77,21 @@ The following values may be used as modifiers.
 
 Less common modifiers might not be supported by your terminal emulator.
 
-| Modifier       |
-| ---            |
-| `bold`         |
-| `dim`          |
-| `italic`       |
-| `underlined`   |
-| `slow_blink`   |
-| `rapid_blink`  |
-| `reversed`     |
-| `hidden`       |
-| `crossed_out`  |
+| Modifier             |
+| ---                  |
+| `bold`               |
+| `dim`                |
+| `italic`             |
+| `underlined`         |
+| `undercurled`        |
+| `underdashed`        |
+| `underdotted`        |
+| `double-underlined`  |
+| `slow_blink`         |
+| `rapid_blink`        |
+| `reversed`           |
+| `hidden`             |
+| `crossed_out`        |
 
 ### Rainbow
 

helix-tui/Cargo.toml

@@ -20,6 +20,7 @@ bitflags = "1.3"
 cassowary = "0.3"
 unicode-segmentation = "1.9"
 crossterm = { version = "0.25", optional = true }
+cxterminfo = "0.2"
 serde = { version = "1", "optional" = true, features = ["derive"]}
 helix-view = { version = "0.6", path = "../helix-view", features = ["term"] }
 helix-core = { version = "0.6", path = "../helix-core" }

helix-tui/src/backend/crossterm.rs

@@ -4,23 +4,56 @@ use crossterm::{
     execute, queue,
     style::{
         Attribute as CAttribute, Color as CColor, Print, SetAttribute, SetBackgroundColor,
-        SetForegroundColor,
+        SetForegroundColor, SetUnderlineColor,
     },
     terminal::{self, Clear, ClearType},
 };
 use helix_view::graphics::{Color, CursorKind, Modifier, Rect};
 use std::io::{self, Write};
 
+fn vte_version() -> Option<usize> {
+    std::env::var("VTE_VERSION").ok()?.parse().ok()
+}
+
+/// Describes terminal capabilities like extended underline, truecolor, etc.
+#[derive(Copy, Clone, Debug, Default)]
+struct Capabilities {
+    /// Support for undercurled, underdashed, etc.
+    has_extended_underlines: bool,
+}
+
+impl Capabilities {
+    /// Detect capabilities from the terminfo database located based
+    /// on the $TERM environment variable. If detection fails, returns
+    /// a default value where no capability is supported.
+    pub fn from_env_or_default() -> Self {
+        match cxterminfo::terminfo::TermInfo::from_env() {
+            Err(_) => Capabilities::default(),
+            Ok(t) => Capabilities {
+                // Smulx, VTE: https://unix.stackexchange.com/a/696253/246284
+                // Su (used by kitty): https://sw.kovidgoyal.net/kitty/underlines
+                has_extended_underlines: t.get_ext_string("Smulx").is_some()
+                    || *t.get_ext_bool("Su").unwrap_or(&false)
+                    || vte_version() >= Some(5102),
+            },
+        }
+    }
+}
+
 pub struct CrosstermBackend<W: Write> {
     buffer: W,
+    capabilities: Capabilities,
 }
 
 impl<W> CrosstermBackend<W>
 where
     W: Write,
 {
     pub fn new(buffer: W) -> CrosstermBackend<W> {
-        CrosstermBackend { buffer }
+        CrosstermBackend {
+            buffer,
+            capabilities: Capabilities::from_env_or_default(),
+        }
     }
 }
 
@@ -47,6 +80,7 @@ where
     {
         let mut fg = Color::Reset;
         let mut bg = Color::Reset;
+        let mut underline = Color::Reset;
         let mut modifier = Modifier::empty();
         let mut last_pos: Option<(u16, u16)> = None;
         for (x, y, cell) in content {
@@ -60,7 +94,7 @@ where
                     from: modifier,
                     to: cell.modifier,
                 };
-                diff.queue(&mut self.buffer)?;
+                diff.queue(&mut self.buffer, self.capabilities)?;
                 modifier = cell.modifier;
             }
             if cell.fg != fg {
@@ -73,6 +107,11 @@ where
                 map_error(queue!(self.buffer, SetBackgroundColor(color)))?;
                 bg = cell.bg;
             }
+            if cell.underline != underline {
+                let color = CColor::from(cell.underline);
+                map_error(queue!(self.buffer, SetUnderlineColor(color)))?;
+                underline = cell.underline;
+            }
 
             map_error(queue!(self.buffer, Print(&cell.symbol)))?;
         }
@@ -135,7 +174,7 @@ struct ModifierDiff {
 }
 
 impl ModifierDiff {
-    fn queue<W>(&self, mut w: W) -> io::Result<()>
+    fn queue<W>(&self, mut w: W, caps: Capabilities) -> io::Result<()>
     where
         W: io::Write,
     {
@@ -153,7 +192,7 @@ impl ModifierDiff {
         if removed.contains(Modifier::ITALIC) {
             map_error(queue!(w, SetAttribute(CAttribute::NoItalic)))?;
         }
-        if removed.contains(Modifier::UNDERLINED) {
+        if removed.intersects(Modifier::ANY_UNDERLINE) {
             map_error(queue!(w, SetAttribute(CAttribute::NoUnderline)))?;
         }
         if removed.contains(Modifier::DIM) {
@@ -166,6 +205,14 @@ impl ModifierDiff {
             map_error(queue!(w, SetAttribute(CAttribute::NoBlink)))?;
         }
 
+        let queue_styled_underline = |styled_underline, w: &mut W| -> io::Result<()> {
+            let underline = match caps.has_extended_underlines {
+                true => styled_underline,
+                false => CAttribute::Underlined,
+            };
+            map_error(queue!(w, SetAttribute(underline)))
+        };
+
         let added = self.to - self.from;
         if added.contains(Modifier::REVERSED) {
             map_error(queue!(w, SetAttribute(CAttribute::Reverse)))?;
@@ -179,6 +226,18 @@ impl ModifierDiff {
         if added.contains(Modifier::UNDERLINED) {
             map_error(queue!(w, SetAttribute(CAttribute::Underlined)))?;
         }
+        if added.contains(Modifier::UNDERCURLED) {
+            queue_styled_underline(CAttribute::Undercurled, &mut w)?;
+        }
+        if added.contains(Modifier::UNDERDOTTED) {
+            queue_styled_underline(CAttribute::Underdotted, &mut w)?;
+        }
+        if added.contains(Modifier::UNDERDASHED) {
+            queue_styled_underline(CAttribute::Underdashed, &mut w)?;
+        }
+        if added.contains(Modifier::DOUBLE_UNDERLINED) {
+            queue_styled_underline(CAttribute::DoubleUnderlined, &mut w)?;
+        }
         if added.contains(Modifier::DIM) {
             map_error(queue!(w, SetAttribute(CAttribute::Dim)))?;
         }

helix-tui/src/buffer.rs

@@ -11,6 +11,7 @@ pub struct Cell {
     pub symbol: String,
     pub fg: Color,
     pub bg: Color,
+    pub underline: Color,
     pub modifier: Modifier,
 }
 
@@ -44,6 +45,9 @@ impl Cell {
         if let Some(c) = style.bg {
             self.bg = c;
         }
+        if let Some(c) = style.underline {
+            self.underline = c;
+        }
         self.modifier.insert(style.add_modifier);
         self.modifier.remove(style.sub_modifier);
         self
@@ -53,6 +57,7 @@ impl Cell {
         Style::default()
             .fg(self.fg)
             .bg(self.bg)
+            .underline(self.underline)
             .add_modifier(self.modifier)
     }
 
@@ -61,6 +66,7 @@ impl Cell {
         self.symbol.push(' ');
         self.fg = Color::Reset;
         self.bg = Color::Reset;
+        self.underline = Color::Reset;
         self.modifier = Modifier::empty();
     }
 }
@@ -71,6 +77,7 @@ impl Default for Cell {
             symbol: " ".into(),
             fg: Color::Reset,
             bg: Color::Reset,
+            underline: Color::Reset,
             modifier: Modifier::empty(),
         }
     }
@@ -97,7 +104,8 @@ impl Default for Cell {
 ///     symbol: String::from("r"),
 ///     fg: Color::Red,
 ///     bg: Color::White,
-///     modifier: Modifier::empty()
+///     underline: Color::Reset,
+///     modifier: Modifier::empty(),
 /// });
 /// buf[(5, 0)].set_char('x');
 /// assert_eq!(buf[(5, 0)].symbol, "x");

helix-tui/src/text.rs

@@ -134,6 +134,7 @@ impl<'a> Span<'a> {
     ///             style: Style {
     ///                 fg: Some(Color::Yellow),
     ///                 bg: Some(Color::Black),
+    ///                 underline: None,
     ///                 add_modifier: Modifier::empty(),
     ///                 sub_modifier: Modifier::empty(),
     ///             },
@@ -143,6 +144,7 @@ impl<'a> Span<'a> {
     ///             style: Style {
     ///                 fg: Some(Color::Yellow),
     ///                 bg: Some(Color::Black),
+    ///                 underline: None,
     ///                 add_modifier: Modifier::empty(),
     ///                 sub_modifier: Modifier::empty(),
     ///             },
@@ -152,6 +154,7 @@ impl<'a> Span<'a> {
     ///             style: Style {
     ///                 fg: Some(Color::Yellow),
     ///                 bg: Some(Color::Black),
+    ///                 underline: None,
     ///                 add_modifier: Modifier::empty(),
     ///                 sub_modifier: Modifier::empty(),
     ///             },
@@ -161,6 +164,7 @@ impl<'a> Span<'a> {
     ///             style: Style {
     ///                 fg: Some(Color::Yellow),
     ///                 bg: Some(Color::Black),
+    ///                 underline: None,
     ///                 add_modifier: Modifier::empty(),
     ///                 sub_modifier: Modifier::empty(),
     ///             },

helix-view/src/graphics.rs

@@ -329,15 +329,25 @@ bitflags! {
     /// ```
     #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
     pub struct Modifier: u16 {
-        const BOLD              = 0b0000_0000_0001;
-        const DIM               = 0b0000_0000_0010;
-        const ITALIC            = 0b0000_0000_0100;
-        const UNDERLINED        = 0b0000_0000_1000;
-        const SLOW_BLINK        = 0b0000_0001_0000;
-        const RAPID_BLINK       = 0b0000_0010_0000;
-        const REVERSED          = 0b0000_0100_0000;
-        const HIDDEN            = 0b0000_1000_0000;
-        const CROSSED_OUT       = 0b0001_0000_0000;
+        const BOLD              = 0b0000_0000_0000_0001;
+        const DIM               = 0b0000_0000_0000_0010;
+        const ITALIC            = 0b0000_0000_0000_0100;
+        const UNDERLINED        = 0b0000_0000_0000_1000;
+        const SLOW_BLINK        = 0b0000_0000_0001_0000;
+        const RAPID_BLINK       = 0b0000_0000_0010_0000;
+        const REVERSED          = 0b0000_0000_0100_0000;
+        const HIDDEN            = 0b0000_0000_1000_0000;
+        const CROSSED_OUT       = 0b0000_0001_0000_0000;
+        const UNDERCURLED       = 0b0000_0010_0000_0000;
+        const UNDERDOTTED       = 0b0000_0100_0000_0000;
+        const UNDERDASHED       = 0b0000_1000_0000_0000;
+        const DOUBLE_UNDERLINED = 0b0001_0000_0000_0000;
+
+        const ANY_UNDERLINE     = Self::UNDERLINED.bits
+                                    | Self::UNDERCURLED.bits
+                                    | Self::UNDERDOTTED.bits
+                                    | Self::UNDERDASHED.bits
+                                    | Self::DOUBLE_UNDERLINED.bits;
     }
 }
 
@@ -355,6 +365,10 @@ impl FromStr for Modifier {
             "reversed" => Ok(Self::REVERSED),
             "hidden" => Ok(Self::HIDDEN),
             "crossed_out" => Ok(Self::CROSSED_OUT),
+            "undercurled" => Ok(Self::UNDERCURLED),
+            "underdotted" => Ok(Self::UNDERDOTTED),
+            "underdashed" => Ok(Self::UNDERDASHED),
+            "double_underlined" => Ok(Self::DOUBLE_UNDERLINED),
             _ => Err("Invalid modifier"),
         }
     }
@@ -426,6 +440,7 @@ impl FromStr for Modifier {
 pub struct Style {
     pub fg: Option<Color>,
     pub bg: Option<Color>,
+    pub underline: Option<Color>,
     pub add_modifier: Modifier,
     pub sub_modifier: Modifier,
 }
@@ -435,6 +450,7 @@ impl Default for Style {
         Style {
             fg: None,
             bg: None,
+            underline: None,
             add_modifier: Modifier::empty(),
             sub_modifier: Modifier::empty(),
         }
@@ -447,6 +463,7 @@ impl Style {
         Style {
             fg: Some(Color::Reset),
             bg: Some(Color::Reset),
+            underline: Some(Color::Reset),
             add_modifier: Modifier::empty(),
             sub_modifier: Modifier::all(),
         }
@@ -482,6 +499,21 @@ impl Style {
         self
     }
 
+    /// Changes the underline color.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// # use helix_view::graphics::{Color, Style};
+    /// let style = Style::default().underline(Color::Blue);
+    /// let diff = Style::default().underline(Color::Red);
+    /// assert_eq!(style.patch(diff), Style::default().underline(Color::Red));
+    /// ```
+    pub fn underline(mut self, color: Color) -> Style {
+        self.underline = Some(color);
+        self
+    }
+
     /// Changes the text emphasis.
     ///
     /// When applied, it adds the given modifier to the `Style` modifiers.
@@ -538,6 +570,7 @@ impl Style {
     pub fn patch(mut self, other: Style) -> Style {
         self.fg = other.fg.or(self.fg);
         self.bg = other.bg.or(self.bg);
+        self.underline = other.underline.or(self.underline);
 
         self.add_modifier.remove(other.sub_modifier);
         self.add_modifier.insert(other.add_modifier);