Remove app mode auto-toggle for small viewports (kernel#155)

## Summary
- Removes the `ensureAppMode` logic that automatically added `--app`
flag and forced Chromium restarts when switching to viewports below
500px width
- The patched kernel-browser removes Chromium's minimum width
constraint, making this workaround unnecessary
- Removes `RemoveFlagsByPrefix`, `HasFlagWithPrefix`, `appModeURL`
constant, and associated tests

## Test plan
- [x] Verified locally with patched Chromium (v145) — 390x844 viewport
works without `--app` mode or restart
- [ ] CI passes

Made with [Cursor](https://cursor.com)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Removes a conditional flag-mutation path during display resizing;
behavior change is localized and simplifies the restart logic, with
minimal impact outside small-viewport scenarios.
> 
> **Overview**
> Removes the *small-viewport “app mode” workaround* in `PatchDisplay`:
resolution changes no longer auto-add/remove the Chromium `--app` flag
(and no longer force a restart solely due to that toggle).
> 
> Cleans up the now-unused support code by deleting `appModeURL`,
`ensureAppMode`, and the `chromiumflags` helpers
`RemoveFlagsByPrefix`/`HasFlagWithPrefix` along with their unit tests;
`chromiumFlagsPath` is kept as a single constant for flag-file
operations.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c47197f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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

Author: hiroTamada

server/cmd/api/api/chromium.go

@@ -21,14 +21,8 @@ import (
 
 var nameRegex = regexp.MustCompile(`^[A-Za-z0-9._-]{1,255}$`)
 
-const (
-	// chromiumFlagsPath is the runtime flags file read by the chromium-launcher at startup.
-	chromiumFlagsPath = "/chromium/flags"
-
-	// appModeURL is the URL loaded in --app mode for small viewports. Keep in
-	// sync with "NewTabPageLocation" in shared/chromium-policies/managed/policy.json.
-	appModeURL = "https://start.duckduckgo.com"
-)
+// chromiumFlagsPath is the runtime flags file read by the chromium-launcher at startup.
+const chromiumFlagsPath = "/chromium/flags"
 
 // UploadExtensionsAndRestart handles multipart upload of one or more extension zips, extracts
 // them under /home/kernel/extensions/<name>, writes /chromium/flags to enable them, restarts

server/cmd/api/api/display.go

@@ -10,7 +10,6 @@ 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"
 )
@@ -89,21 +88,6 @@ 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() {
@@ -407,54 +391,3 @@ func (s *ApiService) setResolutionViaNeko(ctx context.Context, width, height, re
 	return nil
 }
 
-// ensureAppMode adds or removes the --app flag from the Chromium runtime flags
-// file. When enabling, any existing --app flag is removed first so the URL is
-// always exactly appModeURL (defined in chromium.go). 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 appPrefix = "--app"
-	wantFlag := appPrefix + "=" + appModeURL
-
-	existing, err := chromiumflags.ReadOptionalFlagFile(chromiumFlagsPath)
-	if err != nil {
-		return false, fmt.Errorf("failed to read flags file: %w", err)
-	}
-
-	// Always strip any --app/--app=... flags so we can compare cleanly.
-	stripped := chromiumflags.RemoveFlagsByPrefix(existing, appPrefix)
-
-	if enable {
-		updated := append(stripped, wantFlag)
-		// If the exact flag was already present and nothing else changed, no-op.
-		if chromiumflags.HasFlagWithPrefix(existing, appPrefix) && len(updated) == len(existing) {
-			// Check the existing flag is the exact one we want.
-			for _, tok := range existing {
-				if tok == wantFlag {
-					log.Info("app mode already enabled with correct URL, no change needed")
-					return false, nil
-				}
-			}
-		}
-		log.Info("enabling app mode for small viewport", "flag", wantFlag)
-		if err := writeChromiumFlags(updated); err != nil {
-			return false, err
-		}
-		log.Info("app mode toggled", "enabled", true, "flags", updated)
-		return true, nil
-	}
-
-	// Disabling: if nothing was stripped, already disabled.
-	if len(stripped) == len(existing) {
-		log.Info("app mode already disabled, no change needed")
-		return false, nil
-	}
-
-	log.Info("disabling app mode for normal viewport")
-	if err := writeChromiumFlags(stripped); err != nil {
-		return false, err
-	}
-	log.Info("app mode toggled", "enabled", false, "flags", stripped)
-	return true, nil
-}

server/lib/chromiumflags/chromiumflags.go

@@ -224,31 +224,6 @@ 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

@@ -7,8 +7,6 @@ import (
 	"reflect"
 	"strings"
 	"testing"
-
-	"github.com/stretchr/testify/assert"
 )
 
 func TestParseFlags(t *testing.T) {
@@ -216,62 +214,6 @@ 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