2 files changed, 37 insertions, 4 deletions
diff --git a/src/protocol/render_ansi.rs b/src/protocol/render_ansi.rs
index 12606bff..3346bb9c 100644
--- a/src/protocol/render_ansi.rs
+++ b/src/protocol/render_ansi.rs
@@ -31,7 +31,7 @@ use std::io::Write;
 
 use unicode_width::UnicodeWidthStr;
 
-use crate::protocol::{CellData, FrameData};
+use crate::protocol::{underline_style_from_modifier, CellData, FrameData};
 
 /// Bytes produced by a [`BlitEncoder`] for one terminal frame.
 pub(crate) struct EncodedBlit {
@@ -294,7 +294,13 @@ fn modifier_to_sgr_parts(val: u16) -> Vec<&'static str> {
         parts.push("3");
     }
     if val & UNDERLINED != 0 {
-        parts.push("4");
+        parts.push(match underline_style_from_modifier(val) {
+            2 => "4:2",
+            3 => "4:3",
+            4 => "4:4",
+            5 => "4:5",
+            _ => "4",
+        });
     }
     if val & SLOW_BLINK != 0 {
         parts.push("5");
@@ -843,6 +849,18 @@ mod tests {
     }
 
     #[test]
+    fn build_sgr_preserves_curly_underline_style() {
+        let modifier = crate::protocol::modifier_to_u16(
+            crate::protocol::modifier_with_underline_style(ratatui::style::Modifier::UNDERLINED, 3),
+        );
+
+        assert_eq!(
+            build_sgr(0x00_00_00_00, 0x00_00_00_00, modifier),
+            "\x1b[0;4:3;39;49m"
+        );
+    }
+
+    #[test]
     fn cells_equal_identical() {
         let a = make_cell("A", 2, 1, 0);
         let b = make_cell("A", 2, 1, 0);
diff --git a/src/protocol/wire.rs b/src/protocol/wire.rs
index 5f44691b..8225cb8c 100644
--- a/src/protocol/wire.rs
+++ b/src/protocol/wire.rs
@@ -426,7 +426,7 @@ pub struct CellData {
     pub fg: u32,
     /// Background color as a packed u32.
     pub bg: u32,
-    /// Bitmask of style modifiers (bold, italic, etc.).
+    /// Bitmask of style modifiers (bold, italic, etc.) plus Herdr extension bits.
     pub modifier: u16,
     /// Whether this cell should be skipped during diff-based rendering.
     pub skip: bool,
@@ -729,15 +729,30 @@ fn u32_to_color(val: u32) -> ratatui::style::Color {
     }
 }
 
+const UNDERLINE_STYLE_SHIFT: u16 = 12;
+const UNDERLINE_STYLE_MASK: u16 = 0xF000;
+
 /// Converts a ratatui `Modifier` bitmask to a u16 for wire transport.
 pub(crate) fn modifier_to_u16(modifier: ratatui::style::Modifier) -> u16 {
     modifier.bits()
 }
 
+pub(crate) fn underline_style_from_modifier(modifier: u16) -> u8 {
+    ((modifier & UNDERLINE_STYLE_MASK) >> UNDERLINE_STYLE_SHIFT) as u8
+}
+
+pub(crate) fn modifier_with_underline_style(
+    modifier: ratatui::style::Modifier,
+    underline_style: u8,
+) -> ratatui::style::Modifier {
+    let bits = modifier.bits() | ((u16::from(underline_style) & 0x0F) << UNDERLINE_STYLE_SHIFT);
+    ratatui::style::Modifier::from_bits_retain(bits)
+}
+
 /// Converts a u16 back to a ratatui `Modifier`.
 #[cfg(test)]
 fn u16_to_modifier(val: u16) -> ratatui::style::Modifier {
-    ratatui::style::Modifier::from_bits_truncate(val)
+    ratatui::style::Modifier::from_bits_truncate(val & !UNDERLINE_STYLE_MASK)
 }
 
 // ---------------------------------------------------------------------------