Add support for double underlines

This adds support for double underlines using the colon separated escape
sequence `CSI 4 : 2 m`.

Alacritty will now also always fallback to the normal underline in case
any of the other underlines like the undercurl are specified. The escape
sequence `CSI 4 : 0 m` can now be used to clear all underlines.

Some terminals support `CSI 21 m` for double underline, but since
Alacritty already uses that as cancel bold which is a little more
consistent, that behavior has not changed. So the colon separated
variant must be used.

2 files changed, 27 insertions, 16 deletions

diff --git a/alacritty_terminal/src/term/cell.rs b/alacritty_terminal/src/term/cell.rs
index 3fdd8cea..ee95fcba 100644
--- a/alacritty_terminal/src/term/cell.rs
+++ b/alacritty_terminal/src/term/cell.rs
@@ -12,19 +12,20 @@ pub const MAX_ZEROWIDTH_CHARS: usize = 5;
 bitflags! {
     #[derive(Serialize, Deserialize)]
     pub struct Flags: u16 {
-        const INVERSE                   = 0b000_0000_0001;
-        const BOLD                      = 0b000_0000_0010;
-        const ITALIC                    = 0b000_0000_0100;
-        const BOLD_ITALIC               = 0b000_0000_0110;
-        const UNDERLINE                 = 0b000_0000_1000;
-        const WRAPLINE                  = 0b000_0001_0000;
-        const WIDE_CHAR                 = 0b000_0010_0000;
-        const WIDE_CHAR_SPACER          = 0b000_0100_0000;
-        const DIM                       = 0b000_1000_0000;
-        const DIM_BOLD                  = 0b000_1000_0010;
-        const HIDDEN                    = 0b001_0000_0000;
-        const STRIKEOUT                 = 0b010_0000_0000;
-        const LEADING_WIDE_CHAR_SPACER  = 0b100_0000_0000;
+        const INVERSE                   = 0b0000_0000_0000_0001;
+        const BOLD                      = 0b0000_0000_0000_0010;
+        const ITALIC                    = 0b0000_0000_0000_0100;
+        const BOLD_ITALIC               = 0b0000_0000_0000_0110;
+        const UNDERLINE                 = 0b0000_0000_0000_1000;
+        const WRAPLINE                  = 0b0000_0000_0001_0000;
+        const WIDE_CHAR                 = 0b0000_0000_0010_0000;
+        const WIDE_CHAR_SPACER          = 0b0000_0000_0100_0000;
+        const DIM                       = 0b0000_0000_1000_0000;
+        const DIM_BOLD                  = 0b0000_0000_1000_0010;
+        const HIDDEN                    = 0b0000_0001_0000_0000;
+        const STRIKEOUT                 = 0b0000_0010_0000_0000;
+        const LEADING_WIDE_CHAR_SPACER  = 0b0000_0100_0000_0000;
+        const DOUBLE_UNDERLINE          = 0b0000_1000_0000_0000;
     }
 }
 
@@ -58,6 +59,7 @@ impl GridCell for Cell {
             && !self.flags.intersects(
                 Flags::INVERSE
                     | Flags::UNDERLINE
+                    | Flags::DOUBLE_UNDERLINE
                     | Flags::STRIKEOUT
                     | Flags::WRAPLINE
                     | Flags::WIDE_CHAR_SPACER
diff --git a/alacritty_terminal/src/term/mod.rs b/alacritty_terminal/src/term/mod.rs
index 7a409e2e..993e6918 100644
--- a/alacritty_terminal/src/term/mod.rs
+++ b/alacritty_terminal/src/term/mod.rs
@@ -311,7 +311,7 @@ impl RenderableCell {
 
     fn is_empty(&self) -> bool {
         self.bg_alpha == 0.
-            && !self.flags.intersects(Flags::UNDERLINE | Flags::STRIKEOUT)
+            && !self.flags.intersects(Flags::UNDERLINE | Flags::STRIKEOUT | Flags::DOUBLE_UNDERLINE)
             && self.inner == RenderableCellContent::Chars([' '; cell::MAX_ZEROWIDTH_CHARS + 1])
     }
 
@@ -2017,8 +2017,17 @@ impl<T: EventListener> Handler for Term<T> {
             Attr::CancelBoldDim => cursor.template.flags.remove(Flags::BOLD | Flags::DIM),
             Attr::Italic => cursor.template.flags.insert(Flags::ITALIC),
             Attr::CancelItalic => cursor.template.flags.remove(Flags::ITALIC),
-            Attr::Underline => cursor.template.flags.insert(Flags::UNDERLINE),
-            Attr::CancelUnderline => cursor.template.flags.remove(Flags::UNDERLINE),
+            Attr::Underline => {
+                cursor.template.flags.remove(Flags::DOUBLE_UNDERLINE);
+                cursor.template.flags.insert(Flags::UNDERLINE);
+            },
+            Attr::DoubleUnderline => {
+                cursor.template.flags.remove(Flags::UNDERLINE);
+                cursor.template.flags.insert(Flags::DOUBLE_UNDERLINE);
+            },
+            Attr::CancelUnderline => {
+                cursor.template.flags.remove(Flags::UNDERLINE | Flags::DOUBLE_UNDERLINE);
+            },
             Attr::Hidden => cursor.template.flags.insert(Flags::HIDDEN),
             Attr::CancelHidden => cursor.template.flags.remove(Flags::HIDDEN),
             Attr::Strike => cursor.template.flags.insert(Flags::STRIKEOUT),