fix: set default viewport for NewBrowser and align User-Agent with engine

NewBrowser previously had no viewport (strong headless signal) and used a
Firefox User-Agent unconditionally, even for Chromium instances (detectable
mismatch).

Add per-engine UA constants (DefaultFirefoxUserAgent, DefaultChromiumUserAgent)
and auto-select the matching UA in initBrowser when the caller hasn't set one
explicitly. Keep DefaultUserAgent as a backward-compatible alias.

Add 1920x1080 default viewport to NewBrowser (most common desktop resolution).
NewInteractiveBrowser keeps its existing 1280x720 default but also gains
engine-aware UA selection.

Closes #70

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browser_init.go

@@ -52,6 +52,16 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		return nil, ErrInvalidBrowserSelection
 	}
 
+	// Auto-select a User-Agent matching the browser engine when the caller hasn't set one.
+	if opt.UserAgent == "" {
+		switch opt.Browser {
+		case BrowserChromium:
+			opt.UserAgent = DefaultChromiumUserAgent
+		default:
+			opt.UserAgent = DefaultFirefoxUserAgent
+		}
+	}
+
 	// Collect launch args and init scripts, starting with any stealth-mode presets.
 	stealth := opt.Stealth == nil || *opt.Stealth
 	var launchArgs []string

interactive.go

@@ -59,10 +59,9 @@ type interactiveBrowser struct {
 func NewInteractiveBrowser(ctx context.Context, opts ...BrowserOptions) (InteractiveBrowser, error) {
 	var thirtySeconds = 30 * time.Second
 	opt := mergeOptions(BrowserOptions{
-		UserAgent: DefaultUserAgent,
+		Browser: BrowserFirefox,
-		Browser:   BrowserFirefox,
+		Timeout: &thirtySeconds,
-		Timeout:   &thirtySeconds,
+		Stealth: Bool(true),
-		Stealth:   Bool(true),
 		Dimensions: Size{
 			Width:  1280,
 			Height: 720,

playwright.go

@@ -36,8 +36,14 @@ const (
 	BrowserWebKit   BrowserSelection = "webkit"
 )
 
-// DefaultUserAgent is the user-agent string used by all browser instances.
+// DefaultFirefoxUserAgent is the user-agent string used for Firefox browser instances.
-const DefaultUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:147.0) Gecko/20100101 Firefox/147.0"
+const DefaultFirefoxUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:147.0) Gecko/20100101 Firefox/147.0"
+
+// DefaultChromiumUserAgent is the user-agent string used for Chromium browser instances.
+const DefaultChromiumUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36"
+
+// DefaultUserAgent is an alias for DefaultFirefoxUserAgent, retained for backward compatibility.
+const DefaultUserAgent = DefaultFirefoxUserAgent
 
 // Bool returns a pointer to the given bool value.
 func Bool(v bool) *bool { return &v }
@@ -47,7 +53,7 @@ type Size struct {
 	Height int
 }
 type BrowserOptions struct {
-	UserAgent string                     // If empty, defaults to DefaultUserAgent
+	UserAgent string                     // If empty, auto-selected based on Browser engine
 	Browser   BrowserSelection // If unset defaults to Firefox.
 	Timeout   *time.Duration             // If unset defaults to 30 seconds timeout.  If set to 0, no timeout
 
@@ -145,10 +151,13 @@ func playwrightCookieToCookie(cookie playwright.Cookie) Cookie {
 func NewBrowser(ctx context.Context, opts ...BrowserOptions) (Browser, error) {
 	var thirtySeconds = 30 * time.Second
 	opt := mergeOptions(BrowserOptions{
-		UserAgent: DefaultUserAgent,
+		Browser: BrowserFirefox,
-		Browser:   BrowserFirefox,
+		Timeout: &thirtySeconds,
-		Timeout:   &thirtySeconds,
+		Stealth: Bool(true),
-		Stealth:   Bool(true),
+		Dimensions: Size{
+			Width:  1920,
+			Height: 1080,
+		},
 	}, opts)
 
 	if err := ctx.Err(); err != nil {

stealth_test.go

@@ -374,3 +374,68 @@ func TestStealthFirefoxScripts_NoChromiumMarkers(t *testing.T) {
 		}
 	}
 }
+
+// --- User-Agent constants ---
+
+func TestDefaultUserAgent_BackwardCompat(t *testing.T) {
+	if DefaultUserAgent != DefaultFirefoxUserAgent {
+		t.Fatal("DefaultUserAgent must equal DefaultFirefoxUserAgent for backward compatibility")
+	}
+}
+
+func TestDefaultFirefoxUserAgent_Content(t *testing.T) {
+	if !strings.Contains(DefaultFirefoxUserAgent, "Firefox") {
+		t.Fatal("DefaultFirefoxUserAgent must contain 'Firefox'")
+	}
+	if strings.Contains(DefaultFirefoxUserAgent, "Chrome") {
+		t.Fatal("DefaultFirefoxUserAgent must not contain 'Chrome'")
+	}
+}
+
+func TestDefaultChromiumUserAgent_Content(t *testing.T) {
+	if !strings.Contains(DefaultChromiumUserAgent, "Chrome") {
+		t.Fatal("DefaultChromiumUserAgent must contain 'Chrome'")
+	}
+	if strings.Contains(DefaultChromiumUserAgent, "Firefox") {
+		t.Fatal("DefaultChromiumUserAgent must not contain 'Firefox'")
+	}
+}
+
+// --- Viewport and UA defaults via mergeOptions ---
+
+func TestMergeOptions_DefaultViewport(t *testing.T) {
+	base := BrowserOptions{
+		Dimensions: Size{Width: 1920, Height: 1080},
+	}
+	got := mergeOptions(base, nil)
+	if got.Dimensions.Width != 1920 || got.Dimensions.Height != 1080 {
+		t.Fatalf("expected default viewport 1920x1080, got %dx%d", got.Dimensions.Width, got.Dimensions.Height)
+	}
+}
+
+func TestMergeOptions_ViewportOverride(t *testing.T) {
+	base := BrowserOptions{
+		Dimensions: Size{Width: 1920, Height: 1080},
+	}
+	got := mergeOptions(base, []BrowserOptions{{Dimensions: Size{Width: 1280, Height: 720}}})
+	if got.Dimensions.Width != 1280 || got.Dimensions.Height != 720 {
+		t.Fatalf("expected overridden viewport 1280x720, got %dx%d", got.Dimensions.Width, got.Dimensions.Height)
+	}
+}
+
+func TestMergeOptions_EmptyUANotOverridden(t *testing.T) {
+	base := BrowserOptions{}
+	got := mergeOptions(base, []BrowserOptions{{Browser: BrowserChromium}})
+	if got.UserAgent != "" {
+		t.Fatalf("expected empty UserAgent after merge with no explicit UA, got %q", got.UserAgent)
+	}
+}
+
+func TestMergeOptions_ExplicitUAPreserved(t *testing.T) {
+	base := BrowserOptions{}
+	customUA := "MyCustomAgent/1.0"
+	got := mergeOptions(base, []BrowserOptions{{UserAgent: customUA}})
+	if got.UserAgent != customUA {
+		t.Fatalf("expected explicit UA %q preserved, got %q", customUA, got.UserAgent)
+	}
+}