Powerline glyphs being cut for narrow fonts

Fixes alacritty#7470.

CHANGELOG.md

@@ -15,6 +15,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - `chars = "\u000A"` action in bindings inserting `\n`
 - Alternate keys not sent for `Shift + <number>` when using kitty protocol
 - Alternative keys being swapped in kitty protocol implementation
+- Powerline glyphs being cut for narrow fonts
 
 ## 0.13.0
 

alacritty/src/renderer/text/builtin_font.rs

@@ -32,7 +32,7 @@ pub fn builtin_glyph(
         '\u{2500}'..='\u{259f}' => box_drawing(character, metrics, offset),
         // Powerline symbols: '','','',''
         POWERLINE_TRIANGLE_LTR..=POWERLINE_ARROW_RTL => {
-            powerline_drawing(character, metrics, offset)
+            powerline_drawing(character, metrics, offset)?
         },
         _ => return None,
     };
@@ -503,7 +503,11 @@ fn box_drawing(character: char, metrics: &Metrics, offset: &Delta<i8>) -> Raster
     }
 }
 
-fn powerline_drawing(character: char, metrics: &Metrics, offset: &Delta<i8>) -> RasterizedGlyph {
+fn powerline_drawing(
+    character: char,
+    metrics: &Metrics,
+    offset: &Delta<i8>,
+) -> Option<RasterizedGlyph> {
     let height = (metrics.line_height as i32 + offset.y as i32) as usize;
     let width = (metrics.average_advance as i32 + offset.x as i32) as usize;
     let extra_thickness = calculate_stroke_size(width) as i32 - 1;
@@ -519,6 +523,12 @@ fn powerline_drawing(character: char, metrics: &Metrics, offset: &Delta<i8>) ->
     // x = (H - 2) / (2 * slope).
     let x_intersection = (height as i32 + 1) / 2 - 1;
 
+    // Don't use built-in font if we'd cut the tip too much, for example when the font is really
+    // narrow.
+    if x_intersection - width as i32 > 1 {
+        return None;
+    }
+
     let top_line = (0..x_intersection).map(|x| line_equation(slope, x, top_y));
     let bottom_line = (0..x_intersection).map(|x| line_equation(-slope, x, bottom_y));
 
@@ -555,15 +565,15 @@ fn powerline_drawing(character: char, metrics: &Metrics, offset: &Delta<i8>) ->
 
     let top = height as i32 + metrics.descent as i32;
     let buffer = BitmapBuffer::Rgb(canvas.into_raw());
-    RasterizedGlyph {
+    Some(RasterizedGlyph {
         character,
         top,
         left: 0,
         height: height as i32,
         width: width as i32,
         buffer,
         advance: (width as i32, height as i32),
-    }
+    })
 }
 
 #[repr(packed)]