Terminal background: decide 24-bit color like tcell, and log why

The check only looked at COLORTERM, but tcell also draws 24-bit color for
terminals whose description has it (xterm-ghostty, xterm-kitty), so in
Ghostty on macOS without COLORTERM the background was never used. Use
tcell's rules, and always log which background is used and why.

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

internal/ui/app.go

@@ -102,7 +102,7 @@ type overlay struct {
 func New(cfg *config.Config) *App {
 	// Ask the terminal before the screen takes it over and before any widget
 	// copies the default background.
-	ownBackground := useTerminalBackground(terminalAnswers(), os.Getenv)
+	backgroundNote := useTerminalBackground(terminalAnswers(), os.Getenv)
 	a := &App{
 		app:    tview.NewApplication(),
 		tabs:   tview.NewTextView().SetDynamicColors(true),
@@ -139,9 +139,7 @@ func New(cfg *config.Config) *App {
 	if a.covers {
 		a.logs.add(levelInfo, coverNote)
 	}
-	if ownBackground {
-		a.logs.add(levelInfo, "Background: the terminal's own color")
-	}
+	a.logs.add(levelInfo, backgroundNote)
 	sabClient := sabnzbd.NewClient(cfg.SABnzbd.URL, cfg.SABnzbd.APIKey)
 	a.sab = sabClient
 	a.probes = []probe{

internal/ui/terminal.go

@@ -2,6 +2,7 @@ package ui
 
 import (
 	"bytes"
+	"fmt"
 	"image/color"
 	"strconv"
 	"strings"
@@ -9,6 +10,7 @@ import (
 	"time"
 
 	"github.com/gdamore/tcell/v2"
+	"github.com/gdamore/tcell/v2/terminfo"
 	"github.com/rivo/tview"
 )
 
@@ -67,22 +69,42 @@ func parseTerminalAnswers(got []byte) (info terminalInfo) {
 // on which grey text almost disappears). Only for dark backgrounds: the text
 // colors are made for those. Needs true color, else the exact color cannot be
 // drawn. Must run before any widget is created, they copy the style. It
-// reports whether it changed anything.
-func useTerminalBackground(info terminalInfo, env func(string) string) bool {
-	if !info.hasBG || !hasTrueColor(env) || luminance(info.bg) > 0.4 {
-		return false
+// returns a line for the log that says what was decided and why.
+func useTerminalBackground(info terminalInfo, env func(string) string) string {
+	if !info.hasBG {
+		return "Background: palette color 0 (the terminal did not report its background color)"
+	}
+	hex := fmt.Sprintf("#%02x%02x%02x", info.bg.R, info.bg.G, info.bg.B)
+	switch {
+	case !hasTrueColor(env):
+		return "Background: palette color 0 (the terminal's " + hex + " needs 24-bit color, which " + env("TERM") + " does not have; COLORTERM=truecolor enables it)"
+	case luminance(info.bg) > 0.4:
+		return "Background: palette color 0 (the terminal's " + hex + " is light; the text colors are made for dark backgrounds)"
 	}
 	tview.Styles.PrimitiveBackgroundColor = tcell.NewRGBColor(int32(info.bg.R), int32(info.bg.G), int32(info.bg.B))
-	return true
+	return "Background: the terminal's own color " + hex
 }
 
-// hasTrueColor reports whether the terminal says it draws 24-bit colors.
+// hasTrueColor reports whether tcell will draw 24-bit colors, by the same
+// rules: TCELL_TRUECOLOR, COLORTERM, or a terminal description with direct
+// color (e.g. xterm-ghostty and xterm-kitty, also without COLORTERM).
 func hasTrueColor(env func(string) string) bool {
-	switch strings.ToLower(env("COLORTERM")) {
-	case "truecolor", "24bit":
+	switch env("TCELL_TRUECOLOR") {
+	case "":
+	case "disable":
+		return false
+	default:
 		return true
 	}
-	return strings.HasSuffix(env("TERM"), "-direct")
+	switch strings.ToLower(env("COLORTERM")) {
+	case "truecolor", "24bit", "24-bit":
+		return true
+	}
+	if strings.HasSuffix(env("TERM"), "-direct") {
+		return true
+	}
+	ti, err := terminfo.LookupTerminfo(env("TERM"))
+	return err == nil && (ti.TrueColor || ti.SetFgBgRGB != "")
 }
 
 // luminance is the relative brightness of c, from 0 (black) to 1 (white).

internal/ui/terminal_test.go

@@ -2,6 +2,7 @@ package ui
 
 import (
 	"image/color"
+	"strings"
 	"testing"
 
 	"github.com/gdamore/tcell/v2"
@@ -31,6 +32,11 @@ func TestParseTerminalAnswers(t *testing.T) {
 }
 
 func TestUseTerminalBackground(t *testing.T) {
+	// tcell's terminal database also reads these from the real environment
+	// (and keeps what it added to an entry because of them, so the test uses
+	// a terminal nothing else looks up).
+	t.Setenv("COLORTERM", "")
+	t.Setenv("TCELL_TRUECOLOR", "")
 	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}
@@ -44,13 +50,16 @@ func TestUseTerminalBackground(t *testing.T) {
 		want bool
 	}{
 		{"dark background, true color", dark, truecolor, true},
-		{"no true color", dark, env(map[string]string{"TERM": "xterm-256color"}), false},
+		{"no true color", dark, env(map[string]string{"TERM": "vt100"}), false},
+		{"ghostty without COLORTERM", dark, env(map[string]string{"TERM": "xterm-ghostty"}), true},
+		{"true color disabled for tcell", dark, env(map[string]string{"TERM": "xterm-ghostty", "TCELL_TRUECOLOR": "disable"}), 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)
+		note := useTerminalBackground(tc.info, tc.env)
+		if got := strings.Contains(note, "own color"); got != tc.want {
+			t.Errorf("%s: %s", tc.name, note)
 		}
 		want := tcell.ColorBlack
 		if tc.want {