auto cover style: use sextants when fontconfig has a font with the glyphs

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/config/config.go

@@ -43,8 +43,8 @@ type UI struct {
 	// block, e.g. kitty, WezTerm, foot, Ghostty), "kitty" (real images through
 	// the Kitty graphics protocol with Unicode placeholders: kitty and
 	// Ghostty) or "auto" (default: the Kitty protocol where the terminal is
-	// known to support it, else sextant on terminals known to draw them, else
-	// quadrant). The block styles are also the fallback for "kitty".
+	// known to support it, else sextant on terminals known to draw them or when
+	// fontconfig finds a font with the glyphs, else quadrant). The block styles are also the fallback for "kitty".
 	CoverStyle string `json:"cover_style,omitempty"`
 }
 

internal/ui/cover.go

@@ -8,6 +8,7 @@ import (
 	"image/color"
 	_ "image/jpeg" // poster formats
 	_ "image/png"
+	"os/exec"
 	"strings"
 	"time"
 
@@ -138,9 +139,29 @@ func coverStyleFor(setting string, env func(string) string) coverStyle {
 	if env("KITTY_WINDOW_ID") != "" || env("WEZTERM_PANE") != "" {
 		return sextantStyle
 	}
+	// Other terminals (GNOME Terminal, Konsole, Alacritty...) draw glyphs their
+	// font lacks from a fallback font, so sextants work if some installed font
+	// has them. Over SSH the installed fonts are the remote machine's, which
+	// says nothing about the terminal, so keep the safe style there.
+	if env("SSH_CONNECTION") == "" && env("SSH_TTY") == "" && sextantFontAvailable() {
+		return sextantStyle
+	}
 	return quadrantStyle
 }
 
+// sextantFontAvailable reports whether fontconfig knows a font with the
+// sextant characters. It is a variable so tests can replace it.
+var sextantFontAvailable = func() bool {
+	path, err := exec.LookPath("fc-list")
+	if err != nil {
+		return false
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	out, err := exec.CommandContext(ctx, path, ":charset=1fb00", "family").Output()
+	return err == nil && len(bytes.TrimSpace(out)) > 0
+}
+
 // resolveCovers decides how posters are drawn: as real images through the
 // Kitty graphics protocol (graphics) or with block characters in the given
 // style. "kitty" asks for the protocol explicitly; "auto" uses it when the

internal/ui/cover_test.go

@@ -223,7 +223,39 @@ func TestSextantCell(t *testing.T) {
 	}
 }
 
+func TestCoverStyleForFontCheck(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
+
+	sextantFontAvailable = func() bool { return true }
+	for _, tc := range []struct {
+		name string
+		env  map[string]string
+		want string
+	}{
+		{"unknown terminal with a suitable font", map[string]string{"TERM": "xterm-256color"}, "sextant"},
+		{"over ssh the local fonts are unknown", map[string]string{"TERM": "xterm-256color", "SSH_CONNECTION": "1 2 3 4"}, "quadrant"},
+		{"over ssh with a tty", map[string]string{"SSH_TTY": "/dev/pts/1"}, "quadrant"},
+	} {
+		if got := coverStyleFor("auto", env(tc.env)).name; got != tc.want {
+			t.Errorf("%s: %s, want %s", tc.name, got, tc.want)
+		}
+	}
+	// An explicit setting never asks fontconfig.
+	sextantFontAvailable = func() bool { t.Error("fontconfig consulted for an explicit style"); return true }
+	if coverStyleFor("quadrant", env(nil)).name != "quadrant" || coverStyleFor("sextant", env(nil)).name != "sextant" {
+		t.Error("explicit styles must win")
+	}
+	// No suitable font: quadrants.
+	sextantFontAvailable = func() bool { return false }
+	if got := coverStyleFor("auto", env(map[string]string{"TERM": "xterm-256color"})).name; got != "quadrant" {
+		t.Errorf("no font: %s", got)
+	}
+}
+
 func TestCoverStyleFor(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	sextantFontAvailable = func() bool { return false } // the table below is about the terminal checks
 	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
 	for _, tc := range []struct {
 		setting string

internal/ui/kitty_test.go

@@ -143,6 +143,8 @@ func TestPlaceholderCell(t *testing.T) {
 }
 
 func TestKittyTerminalDetection(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	sextantFontAvailable = func() bool { return false } // not what these cases are about
 	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
 	for _, tc := range []struct {
 		name string