Ask the terminal before using Kitty graphics; no fontconfig-based sextants on macOS

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/cover.go

@@ -13,7 +13,9 @@ import (
 	_ "golang.org/x/image/webp"
 	"net/url"
 	"os/exec"
+	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/gdamore/tcell/v2"
@@ -147,7 +149,9 @@ func coverStyleFor(setting string, env func(string) string) coverStyle {
 	// 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() {
+	// macOS terminals do not draw missing glyphs from fontconfig's fonts, so a
+	// font found there says nothing about them.
+	if runtime.GOOS != "darwin" && env("SSH_CONNECTION") == "" && env("SSH_TTY") == "" && sextantFontAvailable() {
 		return sextantStyle
 	}
 	return quadrantStyle
@@ -174,13 +178,27 @@ var sextantFontAvailable = func() bool {
 func resolveCovers(setting string, env func(string) string) (blocks coverStyle, graphics bool) {
 	switch setting {
 	case "kitty":
-		return coverStyleFor("auto", env), true
+		// Asked for explicitly, but a terminal without the protocol prints the
+		// commands as text, so it is still confirmed.
+		return coverStyleFor("auto", env), kittyGraphicsSupported()
 	case "auto", "":
-		return coverStyleFor("auto", env), kittyTerminal(env)
+		return coverStyleFor("auto", env), kittyTerminal(env) && kittyGraphicsSupported()
 	}
 	return coverStyleFor(setting, env), false
 }
 
+var (
+	kittyProbeOnce   sync.Once
+	kittyProbeResult bool
+)
+
+// kittyGraphicsSupported asks the terminal once (see probeKittyGraphics) and
+// remembers the answer. It is a variable so tests can replace it.
+var kittyGraphicsSupported = func() bool {
+	kittyProbeOnce.Do(func() { kittyProbeResult = probeKittyGraphics(300 * time.Millisecond) })
+	return kittyProbeResult
+}
+
 // flatPixelError is the squared RGB distance per pixel below which a cell is
 // drawn flat: smooth areas and noise should not turn into patterns.
 const flatPixelError = 150