Use the terminal's own background color

The panes used palette color 0, which color themes such as Catppuccin set
to a grey lighter than their background: everything looked washed out and
grey text almost disappeared (seen in Ghostty on macOS). The startup probe
now also asks for the background color (OSC 11), and with 24-bit color and
a dark background that color is used. The probe always runs at startup, so
a later switch to real images in the Config view never has to ask the
terminal while the screen owns it.

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

internal/ui/terminal_test.go

@@ -0,0 +1,63 @@
+package ui
+
+import (
+	"image/color"
+	"testing"
+
+	"github.com/gdamore/tcell/v2"
+	"github.com/rivo/tview"
+)
+
+func TestParseTerminalAnswers(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		got      string
+		graphics bool
+		bg       color.RGBA
+		hasBG    bool
+	}{
+		{"ghostty", "\x1b_Gi=31;OK\x1b\\\x1b]11;rgb:1e1e/1e1e/2e2e\x1b\\\x1b[?62;22c", true, color.RGBA{30, 30, 46, 255}, true},
+		{"bel and two digits", "\x1b]11;rgb:ff/80/00\a\x1b[?1;2c", false, color.RGBA{255, 128, 0, 255}, true},
+		{"one digit", "\x1b]11;rgb:f/0/8\x1b\\\x1b[?6c", false, color.RGBA{255, 0, 136, 255}, true},
+		{"only device attributes", "\x1b[?1;2c", false, color.RGBA{}, false},
+		{"garbage", "\x1b]11;rgb:zz/00/00\x1b\\\x1b[?6c", false, color.RGBA{}, false},
+		{"nothing", "", false, color.RGBA{}, false},
+	} {
+		info := parseTerminalAnswers([]byte(tc.got))
+		if info.graphics != tc.graphics || info.hasBG != tc.hasBG || info.bg != tc.bg {
+			t.Errorf("%s: %+v", tc.name, info)
+		}
+	}
+}
+
+func TestUseTerminalBackground(t *testing.T) {
+	defer func(old tcell.Color) { tview.Styles.PrimitiveBackgroundColor = old }(tview.Styles.PrimitiveBackgroundColor)
+	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
+	dark := terminalInfo{bg: color.RGBA{30, 30, 46, 255}, hasBG: true}
+	light := terminalInfo{bg: color.RGBA{239, 241, 245, 255}, hasBG: true}
+	truecolor := env(map[string]string{"COLORTERM": "truecolor"})
+
+	for _, tc := range []struct {
+		name string
+		info terminalInfo
+		env  func(string) string
+		want bool
+	}{
+		{"dark background, true color", dark, truecolor, true},
+		{"no true color", dark, env(map[string]string{"TERM": "xterm-256color"}), false},
+		{"light background keeps the default", light, truecolor, false},
+		{"no answer", terminalInfo{}, truecolor, false},
+	} {
+		tview.Styles.PrimitiveBackgroundColor = tcell.ColorBlack
+		if got := useTerminalBackground(tc.info, tc.env); got != tc.want {
+			t.Errorf("%s: %v", tc.name, got)
+		}
+		want := tcell.ColorBlack
+		if tc.want {
+			want = tcell.NewRGBColor(30, 30, 46)
+		}
+		if tview.Styles.PrimitiveBackgroundColor != want {
+			t.Errorf("%s: background %v", tc.name, tview.Styles.PrimitiveBackgroundColor)
+		}
+	}
+}