feat: auto-toggle Chromium app mode for small viewports

Chromium's minimum window size (~500px wide, ~200px tall) prevents
resizing to mobile viewports like 390x844. Launching with --app mode
removes the tab bar and toolbar chrome, allowing smaller windows.

Changes:
- PatchDisplay now automatically enables --app=https://start.duckduckgo.com
  when width < 500 or height < 200, and disables it for larger sizes
- Add ensureAppMode() to toggle the --app flag in the runtime flags file
  and trigger a Chromium restart when the flag state changes
- Add RemoveFlagsByPrefix and HasFlagWithPrefix helpers to chromiumflags
- Add hasAppFlag helper to chromium-launcher
- Add modelines for 392x844 and 768x1024 at 60/30/25 Hz to xorg.conf
  for mobile and tablet viewport support

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: hiroTamada

images/chromium-headful/xorg.conf

@@ -66,6 +66,10 @@ Section "Monitor"
   Modeline "960x720_60.00" 55.86 960 1008 1104 1248 720 721 724 746 -HSync +Vsync
   # 800x600 @ 60.00 Hz (GTF) hsync: 37.32 kHz; pclk: 38.22 MHz
   Modeline "800x600_60.00" 38.22 800 832 912 1024 600 601 604 622 -HSync +Vsync
+  # 768x1024 @ 60.00 Hz (GTF) hsync: 63.60 kHz; pclk: 65.13 MHz
+  Modeline "768x1024_60.00" 65.13 768 816 896 1024 1024 1025 1028 1060 -HSync +Vsync
+  # 392x844 @ 60.00 Hz (GTF) hsync: 52.44 kHz; pclk: 27.27 MHz (mobile: iPhone 14/15, width rounded from 390)
+  Modeline "392x844_60.00" 27.27 392 416 456 520 844 845 848 874 -HSync +Vsync
   # 2560x1440 @ 60.00 Hz (GTF) hsync: 89.52 kHz; pclk: 312.25 MHz
   Modeline "2560x1440_60.00" 312.25 2560 2752 3024 3488 1440 1443 1448 1493 -HSync +Vsync
   # 1024x768 @ 60.00 Hz (GTF) hsync: 47.70 kHz; pclk: 63.50 MHz
@@ -89,6 +93,10 @@ Section "Monitor"
   Modeline "960x720_30.00" 25.33 960 960 1056 1152 720 721 724 733 -HSync +Vsync
   # 800x600 @ 30.00 Hz (GTF) hsync: 18.33 kHz; pclk: 17.01 MHz
   Modeline "800x600_30.00" 17.01 800 792 864 928 600 601 604 611 -HSync +Vsync
+  # 768x1024 @ 30.00 Hz (GTF) hsync: 31.26 kHz; pclk: 30.01 MHz
+  Modeline "768x1024_30.00" 30.01 768 784 864 960 1024 1025 1028 1042 -HSync +Vsync
+  # 392x844 @ 30.00 Hz (GTF) hsync: 25.77 kHz; pclk: 12.58 MHz (mobile: iPhone 14/15, width rounded from 390)
+  Modeline "392x844_30.00" 12.58 392 400 440 488 844 845 848 859 -HSync +Vsync
   # 1920x1200 @ 30.00 Hz (GTF) hsync: 36.90 kHz; pclk: 96.00 MHz
   Modeline "1920x1200_30.00" 96.00 1920 2000 2200 2528 1200 1203 1209 1235 -HSync +Vsync
   # 1440x900 @ 30.00 Hz (GTF) hsync: 27.72 kHz; pclk: 52.80 MHz
@@ -120,6 +128,10 @@ Section "Monitor"
   Modeline "1200x800_25.00" 31.48 1200 1224 1352 1540 800 801 804 818 -HSync +Vsync
   # 800x1600 @ 24.92 Hz (CVT) hsync: 40.53 kHz; pclk: 41.50 MHz
   Modeline "800x1600_25.00"   41.50  800 832 912 1024  1600 1603 1613 1626 -Hsync +Vsync
+  # 768x1024 @ 25.00 Hz (GTF) hsync: 25.97 kHz; pclk: 24.52 MHz
+  Modeline "768x1024_25.00" 24.52 768 784 856 944 1024 1025 1028 1039 -HSync +Vsync
+  # 392x844 @ 25.00 Hz (manual, min-clock corrected) hsync: 21.42 kHz; pclk: 11.14 MHz (mobile: iPhone 14/15, width rounded from 390)
+  Modeline "392x844_25.00" 11.14 392 408 456 520 844 845 848 857 -HSync +Vsync
 
   # 2560x1440 @ 10.00 Hz (GTF) hsync: 14.65 kHz; pclk: 48.76 MHz
   Modeline "2560x1440_10.00" 48.76 2560 2568 2816 3104 1440 1441 1444 1465 -HSync +Vsync
@@ -143,7 +155,7 @@ Section "Screen"
   SubSection "Display"
     Viewport 0 0
     Depth 24
-    Modes "2560x1440_60.00" "1920x1080_60.00" "1920x1200_60.00" "1440x900_60.00" "1280x720_60.00" "1200x800_60.00" "1152x648_60.00" "1024x768_60.00" "1024x576_60.00" "960x720_60.00" "800x600_60.00" "2560x1440_30.00" "1920x1080_30.00" "1920x1200_30.00" "1440x900_30.00" "1368x768_30.00" "1280x720_30.00" "1200x800_30.00" "1152x648_30.00" "1024x768_30.00" "1024x576_30.00" "960x720_30.00" "800x600_30.00" "800x1600_30.00" "3840x2160_25.00" "2560x1440_25.00" "1920x1080_25.00" "1920x1200_25.00" "1600x900_25.00" "1440x900_25.00" "1368x768_25.00" "1200x800_25.00" "1024x768_25.00" "800x1600_25.00" "2560x1440_10.00" "1920x1080_10.00" "1920x1200_10.00" "1440x900_10.00" "1200x800_10.00" "1024x768_10.00"
+    Modes "2560x1440_60.00" "1920x1080_60.00" "1920x1200_60.00" "1440x900_60.00" "1280x720_60.00" "1200x800_60.00" "1152x648_60.00" "1024x768_60.00" "1024x576_60.00" "960x720_60.00" "800x600_60.00" "768x1024_60.00" "392x844_60.00" "2560x1440_30.00" "1920x1080_30.00" "1920x1200_30.00" "1440x900_30.00" "1368x768_30.00" "1280x720_30.00" "1200x800_30.00" "1152x648_30.00" "1024x768_30.00" "1024x576_30.00" "960x720_30.00" "800x600_30.00" "800x1600_30.00" "768x1024_30.00" "392x844_30.00" "3840x2160_25.00" "2560x1440_25.00" "1920x1080_25.00" "1920x1200_25.00" "1600x900_25.00" "1440x900_25.00" "1368x768_25.00" "1200x800_25.00" "1024x768_25.00" "800x1600_25.00" "768x1024_25.00" "392x844_25.00" "2560x1440_10.00" "1920x1080_10.00" "1920x1200_10.00" "1440x900_10.00" "1200x800_10.00" "1024x768_10.00"
   EndSubSection
 EndSection
 

server/cmd/api/api/display.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	nekooapi "github.com/m1k1o/neko/server/lib/oapi"
+	"github.com/onkernel/kernel-images/server/lib/chromiumflags"
 	"github.com/onkernel/kernel-images/server/lib/logger"
 	oapi "github.com/onkernel/kernel-images/server/lib/oapi"
 )
@@ -88,6 +89,21 @@ func (s *ApiService) PatchDisplay(ctx context.Context, req oapi.PatchDisplayRequ
 		restartChrome = *req.Body.RestartChromium
 	}
 
+	// App mode: Chromium's --app flag removes the tab bar and toolbar chrome,
+	// allowing the window to resize below the normal ~500px minimum width
+	// and ~200px minimum height.
+	// We automatically toggle this based on the requested viewport dimensions.
+	const appModeWidthThreshold = 500
+	const appModeHeightThreshold = 200
+	needsAppMode := width < appModeWidthThreshold || height < appModeHeightThreshold
+	if toggled, err := s.ensureAppMode(ctx, needsAppMode); err != nil {
+		log.Error("failed to toggle app mode", "error", err)
+		// Non-fatal: continue with the resize even if app mode toggle fails.
+	} else if toggled {
+		// App mode changed — force a chromium restart so it picks up the new flags.
+		restartChrome = true
+	}
+
 	// Route to appropriate resolution change handler
 	if displayMode == "xorg" {
 		if s.isNekoEnabled() {
@@ -390,3 +406,47 @@ func (s *ApiService) setResolutionViaNeko(ctx context.Context, width, height, re
 	log.Info("successfully changed resolution via Neko API", "width", width, "height", height, "refresh_rate", refreshRate)
 	return nil
 }
+
+// ensureAppMode adds or removes the --app=about:blank flag from the Chromium
+// runtime flags file. It returns (true, nil) when the flag state was changed
+// (meaning Chromium needs a restart), or (false, nil) when no change was needed.
+func (s *ApiService) ensureAppMode(ctx context.Context, enable bool) (toggled bool, err error) {
+	log := logger.FromContext(ctx)
+	const flagsPath = "/chromium/flags"
+	const appFlag = "--app"
+
+	existing, err := chromiumflags.ReadOptionalFlagFile(flagsPath)
+	if err != nil {
+		return false, fmt.Errorf("failed to read flags file: %w", err)
+	}
+
+	hasApp := chromiumflags.HasFlagWithPrefix(existing, appFlag)
+
+	if enable && hasApp {
+		log.Info("app mode already enabled, no change needed")
+		return false, nil
+	}
+	if !enable && !hasApp {
+		log.Info("app mode already disabled, no change needed")
+		return false, nil
+	}
+
+	var updated []string
+	if enable {
+		log.Info("enabling app mode (--app=about:blank) for small viewport")
+		updated = append(existing, "--app=https://start.duckduckgo.com")
+	} else {
+		log.Info("disabling app mode for normal viewport")
+		updated = chromiumflags.RemoveFlagsByPrefix(existing, appFlag)
+	}
+
+	if err := os.MkdirAll("/chromium", 0o755); err != nil {
+		return false, fmt.Errorf("failed to create chromium dir: %w", err)
+	}
+	if err := chromiumflags.WriteFlagFile(flagsPath, updated); err != nil {
+		return false, fmt.Errorf("failed to write flags file: %w", err)
+	}
+
+	log.Info("app mode toggled", "enabled", enable, "flags", updated)
+	return true, nil
+}

server/cmd/chromium-launcher/main.go

@@ -116,6 +116,16 @@ func main() {
 	}
 }
 
+// hasAppFlag returns true if any token in args is "--app" or starts with "--app=".
+func hasAppFlag(args []string) bool {
+	for _, a := range args {
+		if a == "--app" || strings.HasPrefix(a, "--app=") {
+			return true
+		}
+	}
+	return false
+}
+
 // execLookPath helps satisfy syscall.Exec's requirement to pass an absolute path.
 func execLookPath(file string) (string, error) {
 	if strings.ContainsRune(file, os.PathSeparator) {

server/cmd/chromium-launcher/main_test.go

@@ -5,6 +5,8 @@ import (
 	"os/exec"
 	"path/filepath"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestExecLookPath(t *testing.T) {
@@ -34,3 +36,12 @@ func TestExecLookPath(t *testing.T) {
 		t.Fatalf("execLookPath PATH search failed: p=%q err=%v", p, err)
 	}
 }
+
+func TestHasAppFlag(t *testing.T) {
+	assert.False(t, hasAppFlag([]string{}), "empty args")
+	assert.False(t, hasAppFlag([]string{"--no-first-run", "--disable-gpu"}), "no app flag")
+	assert.True(t, hasAppFlag([]string{"--no-first-run", "--app=about:blank"}), "--app=about:blank")
+	assert.True(t, hasAppFlag([]string{"--app=https://example.com"}), "--app=URL")
+	assert.True(t, hasAppFlag([]string{"--app"}), "bare --app")
+	assert.False(t, hasAppFlag([]string{"--application-name=foo"}), "--application-name should not match")
+}

server/lib/chromiumflags/chromiumflags.go

@@ -224,6 +224,31 @@ func MergeExtensionPath(args []string, extPath string) []string {
 	return result
 }
 
+// RemoveFlagsByPrefix returns a new slice with any tokens that match the given
+// prefix removed. For example, prefix "--app" removes "--app", "--app=about:blank", etc.
+// It does NOT match longer flag names that merely share the prefix
+// (e.g. "--app" will not match "--application-name").
+func RemoveFlagsByPrefix(tokens []string, prefix string) []string {
+	out := make([]string, 0, len(tokens))
+	for _, tok := range tokens {
+		if tok == prefix || strings.HasPrefix(tok, prefix+"=") {
+			continue
+		}
+		out = append(out, tok)
+	}
+	return out
+}
+
+// HasFlagWithPrefix returns true if any token equals prefix or starts with prefix + "=".
+func HasFlagWithPrefix(tokens []string, prefix string) bool {
+	for _, tok := range tokens {
+		if tok == prefix || strings.HasPrefix(tok, prefix+"=") {
+			return true
+		}
+	}
+	return false
+}
+
 // WriteFlagFile writes the provided tokens to the given path as JSON in the
 // form: { "flags": ["--foo", "--bar=1"] } with file mode 0644.
 // The function creates or truncates the file.

server/lib/chromiumflags/chromiumflags_test.go

@@ -7,6 +7,8 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestParseFlags(t *testing.T) {
@@ -214,6 +216,62 @@ func TestWriteFlagFileAndReadBack(t *testing.T) {
 
 // TestWriteFlagFileFromString removed: callers should use WriteFlagFile with tokens.
 
+func TestRemoveFlagsByPrefix(t *testing.T) {
+	tests := []struct {
+		name   string
+		tokens []string
+		prefix string
+		want   []string
+	}{
+		{
+			name:   "remove exact match",
+			tokens: []string{"--foo", "--app", "--bar"},
+			prefix: "--app",
+			want:   []string{"--foo", "--bar"},
+		},
+		{
+			name:   "remove prefix=value match",
+			tokens: []string{"--foo", "--app=about:blank", "--bar"},
+			prefix: "--app",
+			want:   []string{"--foo", "--bar"},
+		},
+		{
+			name:   "does not remove longer flag names",
+			tokens: []string{"--foo", "--application-name=test", "--app=about:blank"},
+			prefix: "--app",
+			want:   []string{"--foo", "--application-name=test"},
+		},
+		{
+			name:   "nothing to remove",
+			tokens: []string{"--foo", "--bar"},
+			prefix: "--app",
+			want:   []string{"--foo", "--bar"},
+		},
+		{
+			name:   "empty input",
+			tokens: []string{},
+			prefix: "--app",
+			want:   []string{},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := RemoveFlagsByPrefix(tt.tokens, tt.prefix)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("RemoveFlagsByPrefix() = %#v, want %#v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestHasFlagWithPrefix(t *testing.T) {
+	assert.True(t, HasFlagWithPrefix([]string{"--app=about:blank", "--foo"}, "--app"))
+	assert.True(t, HasFlagWithPrefix([]string{"--foo", "--app"}, "--app"))
+	assert.False(t, HasFlagWithPrefix([]string{"--application-name=test"}, "--app"))
+	assert.False(t, HasFlagWithPrefix([]string{"--foo", "--bar"}, "--app"))
+	assert.False(t, HasFlagWithPrefix([]string{}, "--app"))
+}
+
 func TestMergeFlags(t *testing.T) {
 	tests := []struct {
 		name         string